In the United States, for most of the history of broadcasting, there were only three or four major commercial national broadcast networks. From 1946-1956 these were ABC, CBS, NBC, and DuMont. From 1956-1986, the national commercial networks were ABC, CBS, and NBC.
Today, more than 50 nationwide broadcasting networks exist. Other than the non-commercial educational (NCE) Public Broadcasting Service (PBS), which is composed of member stations, the largest broadcast television networks are the traditional Big Three Television Networks (ABC, CBS, and NBC). Many other large networks exist, however, notably Fox and The CW which air original programming for two hours each night instead of three like the original "Big Three" do, as well as syndication services like MyNetworkTV and ION which feature reruns of recent popular shows with little or no original programs. Fox has just about the same percentage of households reached as the Big Three, and is therefore often considered a peer to ABC, NBC, and CBS since it has also achieved equal or better ratings over the last decade. Most media outlets now include Fox in what they refer to as the "Big Four" television networks.
The transition to digital broadcasting in 2009 has allowed for television stations to offer more programming options through digital subchannels. A number of new commercial networks airing specialty programming such as movies, reruns of classic shows and lifestyle programs have been created from companies like Weigel Broadcasting, Luken Communications and the major networks. There have also been a number of new Spanish-language networks that have launched as well as some new non-commercial public television channels.
Broadcast networks in the United States can be divided into four categories:
Each network sends its signal to many local affiliatetelevision stations across the country. These local stations then air the "network feed," and millions of households across the country tune in. In the case of the largest networks, the signal is sent to over 200 stations. In the case of the smallest networks, the signal may be sent to just a dozen or fewer stations.
There are an estimated 115.6 million television households in the United States as of the 2013-2014 TV season. 
English-language American commercial over-the-air television networks
ABC (originally American Broadcasting Company; originally formed from the NBC Blue Network of radio, which the U.S. Federal Communications Commission (FCC) forced NBC to sell in 1943) - The nation's third-largest commercial network, Disney-owned ABC has over 200 affiliate stations and airs original programming, sports, and news seven days a week. Almost all ABC stations air local newscasts.
CBS (originally Columbia Broadcasting System) - The nation's second-largest commercial network and owned by CBS Corporation, CBS has over 200 affiliate stations and airs original programming, sports and news seven days a week. Almost all CBS stations air local newscasts. For most of its existence, CBS has been the nation's most watched network.
NBC (originally National Broadcasting Company, and formerly the television sister of the NBC Red Network from the broadcaster's radio days) - The nation's largest and oldest commercial network, owned by NBCUniversal, NBC has over 200 affiliate stations and airs original programming, sports and news seven days a week. Almost all NBC stations air local newscasts.
FOX (Fox Broadcasting Company) - The nation's fourth-largest commercial network, the 21st Century Fox-owned Fox has nearly 200 affiliate stations and airs original programming and sports seven days a week, programming two hours each night in primetime (three hours on Sundays), along with political talk program Fox News Sunday on Sunday mornings and the optional infomercial block Weekend Marketplace on Saturday mornings. Almost all Fox stations air local newscasts, with some producing newscasts in-house and others airing newscasts produced by another station in the market. Several of the network's owned-and-operated stations formed the hub of the DuMont Network, which existed from the late 1940s to the mid-1950s.
The CW (originally formed from The WB and UPN networks, both of which folded in September 2006 after 11 years of existence) - The nation's fifth-largest commercial network, The CW has approximately 100 affiliate stations in the top 100 television markets; it also has approximately 90 additional affiliates operated as cable-only and digital subchannel services in smaller television markets through The CW Plus. The network airs two hours of original programming in primetime and one hour in daytime on Monday through Fridays, and a five-hour children's program block called Vortexx on Saturday mornings. Unlike its four larger major network competitors, The CW does not have owned-and-operated stations in New York City, Los Angeles and Chicago, the affiliates in those markets are owned by Tribune Broadcasting; despite joint ownership by CBS Corporation and Time Warner, CBS Corporation serves as the network's de facto O&O station group (Time Warner owns one station in Atlanta through its Turner Broadcasting System subsidiary, however it does not serve as the CW affiliate for the market). Some CW affiliates air local newscasts, most of which are produced by another station in the market though ten of its affiliates (most of which are owned by Tribune Broadcasting) produce local newscasts in-house.
MyNetworkTV is a network owned by 21st Century Fox, which also owns FOX. It was hastily formed to provide programming for stations left empty-handed by the merger of The WB and UPN. The network launched with English-language telenovelas, but switched to a mix of low-budget programming within the first year. Currently, MyNetworkTV fills two hours of primetime Monday through Fridays with reruns of shows that originated on other broadcast and cable networks. Some MyNetworkTV affiliates air local newscasts, most of which are produced by another station in the market, though San Francisco affiliate KRON-TV is the only affiliate that produces its local news programming in-house.
Me-TV (Memorable Entertainment Television) is large digital network from Weigel Broadcasting that airs reruns of classic television shows. Me-TV airs mostly on digital subchannels but they also have several affiliates in which they are the stations' primary network. Currently covering about 84% of broadcast homes with over 150 affiliates Me-TV is the nations seventh-largest commercial broadcast network.
This TV is a large network airing mostly on digital subchannels. It was launched on November 1, 2008 by the MGM studios and Chicago based Weigel Broadcasting. This TV airs a 24/7 schedule consisting mostly of movies but also a few TV shows. Based on coverage of about 82% and over 110 affiliates, This TV is currently the eighth-largest commercial broadcast network in the United States. However the network may not have the same access to many cable and satellite systems that other broadcast networks would.
Ion Television is a mid-sized network, which airs off-network repeats of shows from other networks and feature films for thirteen hours every day. It is the largest English-language network to wholly handle programming responsibilities for its affiliates, though some affiliates run some locally-produced public affairs and religious programming. Affiliates number around 70, though most of the stations are owned by parent company Ion Media Networks. It aired original programming in the past, and was known as PAX TV from 1998–2005, and i: Independent Television from 2005-2007. Ion owned and operated stations also carry the children's network Qubo and lifestyle channel Ion Life on their digital subchannels, and many of the network's owned stations also carry QVC and the Home Shopping Network on additional subchannels.
America One (a successor to Channel America) is a small network of about 40 affiliates. It airs general entertainment programming (wholly scheduled by the network for its affiliates) with a heavy emphasis on primetime sports programming and events.
Additionally, several of the cable-oriented theme channels (e.g. music or shopping channels) have obtained broadcast clearances, usually on low-power stations, in many markets. Among these are Home Shopping Network, and ShopHQ.
TV Scout is a subchannel network for over the air program grid launched in July 2013 and is available on 12 subchannels as of October 2013.
PBS: "Head Network" for their educational programming, mascot, and logo
UPN defunct: "Used Parts Network" for its purchase of new episodes of series formerly seen on other networks, "Shapes network" or "Disc network" (after the network's 1995-2002 and 2002-2006 logos)
The WB defunct: "Frog network" or the network's secondary branding "The Frog" (after the network's frog mascot)
DuMont Television Network defunct: "The Forgotten Network" (due to its modern-day obscurity, considering it was a major network during the 1940s to 1950s)
Additionally, both The WB and UPN were referred to as weblets by Variety because of their smaller audiences and fewer programs. CW and MyNet have more often been called netlets, which has the same definition.
Spanish-language American commercial over-the-air television networks
Univisión is the nation's largest commercial Spanish-language network owned by Univision Communications, Univision has over 120 affiliate stations including over 35 full-power stations; many Univision owned-and-operated and airs original programming, as well as imported programming from Mexico and Venezuela seven days a week. Many areas of the country without access to a Univision station can receive the network's national feed via cable and satellite. Formed in 1986 following the sale of predecessor Spanish International Network (SIN) to Hallmark from Mexico's Televisa due to federal laws against foreign ownership of American television networks. Most Univision stations air local newscasts.
Telemundo is the nation's second-largest commercial Spanish-language network operated by NBC Universal, Telemundo has over 100 affiliate stations including 18 full-power stations and airs original programming seven days a week. A big amount of its programming is recorded in Miami where the station is headquartered. In addition, Telemundo operates in Mexico and Puerto Rico (where it was founded in 1954). Most Telemundo stations air local newscasts, as well as imported programming from Colombia and to a lesser extent Brazil.
UniMás, known as Telefutura from its founding in January 2002 until January 2013, is the third-largest commercial Spanish-language network, owned by Univision Communications, UniMás has nearly 45 affiliate stations including 35 full-power stations and airs original and imported programming seven days a week.
MundoFox is the nation's fourth-largest commercial Spanish-language network operated as a joint venture between News Corporation subsidiary Fox International Channels and Colombian broadcaster RCN Televisión. MundoFox has over 60 affiliate stations, consisting of mostly low-power stations with some full-power affiliates, and some carriage as a simulcast subchannel by Fox and MyNetworkTV O&O's in major markets due to said low-power and cable carriage limitations. Original and imported programming airs seven days a week, with some of the imported programs distributed by RCN and NTN24. Launched in August 2012, some MundoFox stations air local newscasts, with many others planning to produce local news programming within a year of the network's launch.
Estrella TV is the fifth-largest commercial Spanish-language network, owned by Liberman Broadcasting, Estrella TV has nearly 35 affiliate stations, most of which are owned by Liberman or carry the network as a subchannel-only affiliation, along with cable carriage nationally via some cable providers. The network airs original and imported programming seven days a week for 18 hours each weekday and 11 hours per day on weekends.
Azteca América is the nation's sixth-largest commercial Spanish-language network, Azteca America has nearly 90 affiliate stations including 8 full-power stations and airs original and imported programming seven days a week. Azteca America is an off-shoot of Mexico's Azteca and is owned by Azteca International Corporation, though much of the American network's programming airs at different times.
Exitos TV is a small Spanish-language subchannel network broadcasting on 15 of Telemundo owned and operated TV station's subchannel televising telenovelas.
Additionally, Televisa, which distributes programming to Univision in the United States, operates in Mexico, but their networks (Canal de las Estrellas, Canal 5, and Galavisión) have certain stations which can be seen in areas of the U.S. along and near the Mexican border, and likewise with the American networks in border cities towards Mexico.
Some Mexican border stations (such as Tijuana's XETV-TV) are affiliates of American networks and target their American border city more than their Mexican metropolitan area, broadcasting in English or Spanish, depending on network.
All Spanish-language broadcast networks operate a national feed carried on cable and satellite systems where an affiliate is not present. Univision has a larger cable-only distribution than the other commercial Spanish-language networks and therefore may be carried in more homes than the amount listed in the above table. Spanish-language independent stations also exist, though they are mainly limited to large markets.
The Spanish-language networks have a smaller amount of affiliates than "The Big Three" English-language networks (NBC, ABC and CBS) and PBS, though they still occupy a large share of the country and with a growing Latino population, the number of Spanish-language network affiliates in the United States has increased.
Unlike the English-language broadcast networks, Univision, Telemundo, Azteca America and UniMás do not rely on their affiliate stations to program the majority of the broadcast day as the networks themselves are wholly responsible for handling programming for its affiliates. However, affiliates are allowed to break away from the network feed to offer some locally-produced programming (mainly consisting of local newscasts or lifestyle programming).
Public Broadcasting Service (PBS) - The largest public broadcasting network, somewhat decentralized, in the U.S. PBS also has 24-hour/7-day program feeds that some broadcast (both analog and digital) stations subscribe, for some or all of their dayparts: PBS Satellite Service (which has Eastern and Pacific Time feeds, and was originally conceived as a cable channel for areas not served by PBS stations), PBS YOU or "Your Own University" (devoted largely to adult education, crafts, and public-affairs programming, which ceased operations at the end of January 2006), PBS Kids Channel (devoted to PBS's very popular children's programming; discontinued in favor of a commercial-partnership service called PBS Kids Sprout on October 1, 2005 and briefly meant to be succeeded by a new service PBS Kids Go!, in October 2006), PBS DT2, PBS's HDTV feed of high-definition and letterboxed standard-definition programming, and PBS World, a cooperative effort at a news, public affairs, and documentary service between PBS, American Public Television, and other public-broadcasting entities and headquartered at PBS member station WGBH.
NYCTV - The broadcasting service of the City of New York, offering original Emmy-award winning programming and available nationally on PBS stations. Not a network, but a provider of programming to several New York services beyond the home station, WNYE-TV.
The Annenberg Channel (formerly Annenberg/CPB Channel) - A national educational access channel for public broadcasters and schools it is available on some cable and satellite packages and is one of the only television channels in the U.S with an online stream programming that is offered to broadcast stations and cable systems for carriage; many of the broadcast affiliates play its programming in overnights. It shared some programming with PBS YOU and various university and college stations around the country.
Deutsche Welle (DW TV) A German non-commercial television service which provides some English-language news programming to public broadcasting stations, and whose programming feed can be seen on a small number of independent public-broadcasting stations for part of their broadcast day.
Create (sometimes given as Create!), a 24-7 digital-signal network run by syndicator American Public Television in partnership with WGBH, WNET, WLIW, NETA, and PBS, offering crafts and travel programming in part to fill the void left by the shuttering of PBS YOU. The network began transmission in January, 2006, primarily with a number of digital broadcast stations around the U.S.
AZN Television - Formerly branded as International Channel with a mix of international programming before the rise of digital cable and satellite services allowed carriage of many foreign networks; the AZN iteration offered programming aimed at English-speaking Asian-Americans; operated from 1996 to April 9, 2008.
Badger Television Network - Short-lived television network consisting of three stations in Wisconsin; operated from January to August 1958.
NBC Weather Plus - National digital multicast weather network owned by NBCUniversal; operated from November 15, 2004 to December 31, 2008; some affiliates replaced the service with an automated local weather channel under the name NBC Plus.
Network One (N1) - A small independent network, similar to PAX/i and America One; operated from the mid-1990s to November 13, 1997.
Overmyer Network (ON; launched as the United Network, not to be confused with UPN) - Short-lived commercial network, operated from May 1 to June 1, 1967; .
PBS Kids - Operated from 1999 to 2005; some of its functions are being taken up by successor PBS Kids Sprout, a commercial cable venture, while some individual PBS member stations and networks program 24/7 kids schedule subchannels independently. A new successor service, PBS Kids Go!, was promised for October 2006, but never launched. The PBS Kids name remains as a block branding for PBS children's content.
PBS YOU - Operated from the late 1990s to 2006; "Your Own University," this service offered a mix of crafts, college-credit and Standard Deviants instructional programs, and news, commentary, and documentary programming. Many of its affiliates joined Create, a similar if more craft- and travel-focused service administered by American Public Television, as YOU went dark in early 2006.
SOI TV was a small Spanish-language network started by a Venzezualan banker who was a political prisoner of Chavez he was granted U.S. asylum. The network used interactive broadcast technologies allowing real time response by viewers regarding its television content via Twitter and Facebook. SOI was launched in March 2012 with a $20 million investment. In December, La Familia cable network agreed to carry SOI programming. NBCUniversal owned Telemundo stations carried SOI on a subchannel until January 2013.
TuVisión - Spanish-language commercial network, operated from 2007 to 2009.
TVS Television Network - Sports programming syndicator; peak years were in the 1960s and 1970s. Limited programming continued from then onward until the company ceased operations in 2012.
UPN (formerly initialism for "United Paramount Network") - Operated from January 16, 1995 to September 15, 2006, merged with WB to make The CW.
Universal Sports (formerly World Championship Sports Network) - Operated from 2005 to December 31, 2011; offered Olympic-style sports programming that aired on as many as 56 digital subchannels including all NBC owned-and-operated stations. Network transitioned into a cable and satellite-only channel on January 1, 2012.
Urban America Television (UATV) - A successor to the American Independent Network; a small network with 60 affiliate stations, UATV aired original programming mixed with older films, rather similarly to America One; Operated from December 3, 2001 to May 1, 2006.
^ abcAlthough ABC, NBC, and CBS were founded prior to 1946, those companies did not begin continuous over the air TV broadcasting until 1946 (NBC) and 1948 (ABC and CBS).
^ abDuMont's relationship with the Fox network via Metromedia is disputed, with Fox being either a modern re-incarnation, or at least a linear descendant of the DuMont Television Network (via former DuMont subsidiary Metromedia)
^ abOn January 24, 2006, CBS and Time Warner announced the merger of The WB and UPN, forming one larger network, The CW in late 2006. See The CW for more information. The CW and MyNetworkTV are also carried on cable-only channels and digital subchannels of many currently operating television stations of several different affiliations, ranging from current WB and Fox affiliates, to even NBC and CBS affiliates. The WB and UPN shut down on September 18, 2006 to merge into The CW. MyNetwork TV was created by the Fox Broadcasting Company to give programming to several Fox-owned UPN affiliates, upon the shutdown of UPN.
^Although PBS was initially established in 1969, it assumed full-time broadcasts on October 5, 1970 to replace its predecessor, National Educational Television (NET).
^Cornerstone Television isn't considered a network in the traditional definition of a TV network, rather it is a distributor of in-house produced programming. Although the main station, WPCB-TVPittsburgh is on many owned and operatedtranslators, its Full Power and LPTV "affiliates" typically air one or two Cornerstone Television produced programs per week scheduled at different times than the main Corerstone station and do not identify themselves as "Cornerstone Television" affiliates.
^Although Cornerstone Television was founded in 1970, the network did not begin even limited broadcasts until 1979.