Some 80 Japanese Type D ("Koryu") Midget Submarines in a dry dock at Kure, October 19, 1945
A midget submarine (also called a mini submarine) is any submarine under 150 tons typically operated by a crew of one or two but sometimes up to 6 or 8, with little or no on-board living accommodation. Midget submarines normally work with mother ships, from which they are launched and recovered, and which provide living accommodation for the crew and other support staff.
Both military and civilian midget submarines have been built. Military types work with surface ships and other submarines as mother ships. Civilian and non-combatant military types are generally called submersibles, and normally work with surface ships.
Most early submarines, such as the United States Navy's Holland and the BritishRoyal Navy's Holland 1, would now be considered midget submarines. “Midget subs were not named but were numbered with "Ha" numbers (e.g., Ha-19).[who?] These numbers were not displayed on the exterior and operationally the midgets were referred to according to the numbers of their mother ships. Thus, when I-24 launched Ha-19, the midget was known as "I-24tou" (designated "M24" in some texts). The "Ha" numbers were not unique either; some Type D's were numbered Ha-101 through Ha-109 and SS-Type medium submarines used the same numbers.” 
The best known role for midget submarines is probably harbor penetration, although only two World War II boats, the British X-craft and the unsuccessful Welman submarine were specifically designed with this in mind. Japan's Ko-hyoteki class submarines were originally designed to take part in decisive fleet actions. However, as circumstances changed, they ended up tasked with harbor penetration. Germany’s various World War II designs were mostly designed to attack Allied shipping off landing beaches and harbors, although the Seehund had a great enough range to attack shipping off the Thames estuary.
Midget submarines have also seen some use in support roles. X-craft were used for reconnaissance, and the Seehund was used to carry supplies. A number of modern midget submarines have also been built for submarine rescue.
World War II In the attack on the U.S. Fleet the Japanese used their secret mini-subs at the start of World War II. “They were six feet wide and eighty feet long. Powered by a 600 hp engine they could travel at 19 knots – twice the speed of the submarines that piggybacked them to Pearl Harbor. Each mini-sub carried two 1000 lb torpedoes.” 
“During WWII, the Japanese Navy deployed hundreds of different sorts of submarines, but on December 7th, 1941, they debuted a particularly secret weapon—the Type A Ko-Hyoteki midget sub. Five furtively made their way toward Pearl Harbor to attack U.S. battleships. At a quarter the size of common fleet subs, the midgets were a technological marvel, far outperforming their western counterparts.” 
Aurora class one-person midget submarine: Aurora (Shuguang, 曙光) midget sub is a Chinese civilian midget sub developed by private firm WuhanZhang Wu-Yi Shipyard (武汉张五一船舶厂). Length: 3.6 m, beam: 1.8 m, height: 1.2 m, maximum speed: 20 km/hr, endurance: 10 hr maximum, maximum depth: 20 m. Equipped with a robotic arm and search light to harvest sea cucumbers in mariculture, but can also be deployed for maintenance of underwater structures in harbors/ports. Several foreign countries including North Korea have shown interests and invited the developer to their respective countries to develop similar midget subs, including possible military applications.
Aurora class two-people midget submarine: successor of Aurora class one-person midget sub, with dimension increased to 7 m x 2 m x 2 m from the original 3.6 m x 1.8 m x 1.2 m, and the maximum depth is increased to 30 m, while speed and endurance remained the same. Crew is increased to two. As with its predecessor, this two-people version also attracted attention of prospective foreign interests.
Tao Xiangli class midget submarine: This homemade submarine is developed by Mr. Tao Xiangli (陶相礼, 1974 -). To reduce developmental cost, Mr. Tao uses 2nd hand material whenever it was possible, and as result, it only took ¥ 30,000 (US$ 4,385) to complete his first boat. For example,the hull is constructed of five used fuel tanks. Tao's submarine weighs 800 kg (1,764 lb) and is 6.5 meters (21 feet) long, with a cramped interior that fits one person and features pressure gauges, monitoring cameras and, of course, an oxygen supply. The electrically powered midget sub shapes like a torpedo with a conning tower, and is able to dive to 10 meter depth. Mr. Tao confirmed that he has received funds for further development, which would enable the next midget sub planned to build to dive to a depth of 50 meters.
2 Italian COS.MO.S CE2F/X100 post-war torpedo chariots
2 Italian COS.MO.S S.X.404 midget submarines: S-1 Haijiao (海蛟), S-2 Hailong (海龍), in service from October 8, 1969 to November 1, 1973. S.X.404 is a 70 ton 4-man crew submarine design by Cosmos. Four were sold to Colombia and two to Taiwan in the 1970s. Displaced is 40/70 t (surfaced/submerged), speed is 11/6.5 kn (surfaced/submerged), range is 1200/60 nmi (surfaced/submerged) @ respective maximum speed. Payload included externally attached torpedoes or mines, or swimmer delivery vehicles.
S.X.506 a 70 ton 5-man crew midget submarine designed by Italian shipbuilder Cosmos as a development of earlier S.X.404 by the same firm. Displacement is 58/70 t (surfaced/submerged), length is 23 m, beam is 2 m, draft is 4 m, and speed is 8.5/7 kn (surfaced/submerged). At a speed of 7 kn, range is 1200/60 nmi (surfaced/submerged). Payload includes externally attached torpedoes and swimmer delivery vehicles, and up to 8 divers can be carried in addition to the 5-man crew.
Most German midget submarines were developed late in World War II in an attempt to stop the Allied invasion of Europe and used later to disrupt its supply lines. As a result, the submarines mostly engaged in open water attacks rather than harbour penetration. "1154 U-boats were commissioned into the Kriegsmarine before and during World War Two." 
Biber (324 built by AG Weser of Bremen) "324 of these small craft were built and many saw action." 
Delphin (2 built) 2-man 5-ton torpedo with top speed of 20 knots (37 km/h) and submerged radius of 30 nautical miles (56 km) at 3 knots (5.6 km/h). "Designed as fast midget boat. 3 prototypes were built."
Hecht type XXVIIA 2-man 12 ton submarine with 1 mine or 1 torpedo carried outboard to a range of 38 miles at 4 knots. "All of the 53 boats of this type were used for training only." (Uboat.net) 
Marder (~300 built) similar to Neger with breathing apparatus to allow submerged operation. "About 500 of these boats were constructed until May 1945." 
Molch Completely electric motordriven WWII (1944–1945) Midget submarine. Total 393 built. "393 of these all-electric midget boats were produced."
Schwertwal (Grampus) Designed as super-fast Walter-type midget boat, but the prototype was not complete when scuttled in May 1945.
Neger (~200 built) 1-man 5-ton torpedo with underslung G7E torpedo. Top speed 20 knots and range of 30 miles at 3 knots. "Just about 200 boats of this type were built."
Seehund type XXVIIB Most successful midget submarine in the Kriegsmarine. Operational deployment was between January 1945 – April 1945. "The most successful of the midget boats, 285 built and 138 commissioned."
V.80 4-man 76-ton prototype completed in 1940 to test Walther geared turbine propulsion system. Range was 50 nautical miles (93 km) at 28 knots (52 km/h).
Orca class post-war midget submarine: 13.27 meter x 2.1 meter 5 men midget submarine with maximum depth of 100, maximum underwater speed of 5 kn, maximum range of 150 nmi, maximum endurance of 4 days, and maximum payload of 250 kg. Periscopes includes television camera, HF/HF communication and GPS antenna. There is also an underwater television camera and obstacle avoidance sonar with an effective range of 200 m. The low acoustic and magnetic signature midget sub displaces 28 t, powered by two computerized permanent magnetic electric motors that drive a single shaft low-cavitation and low noise propeller, which is controlled by fly-by-wire control system that is usually adopted for aircraft. The pressure hull is a modular design consisted of four modules that enables easy transportation and maintenance.
The Indonesian Navy has shown some interest in having a new Midget Class submarine, built by local shipyards, for coastal rather than open water patrol. The submarine was designed a number of years ago by a retired Indonesian Navy submariner officer, Colonel (Ret) Ir. R. Dradjat Budiyanto, Msc. The midget experiment project involves the construction of a submarine, designated MIDGET IM X −1, which will weigh about 150 – 250 tonnes, with a tubular frame design 24 – 30 meters long, and four torpedo tubes. The submarines will have minimum of 8 – 10 crew members including officers. They will have a 40 km range non-hull-penetrating optronic mast as the attack periscope, and a 20 km range navigation periscope.
Indonesian defence minister Purnomo Yusgiantoro has backed the project. Construction should commence at the Indonesian PT.PAL INDONESIA shipyard by late 2011, and will take about three or four years to complete. If this schedule is met, the Indonesian Navy expects to be commissioning the first Midget Class submarine in 2014.
Siluro a Lenta Corsa (SLC), also known as Maiale (pig), a low speed human torpedo. On December 3, 1941, the Sciré departed La Spezia carrying three Maiale to conduct what became the Raid on Alexandria (1941). At the island of Leros in the Aegean Sea, six Decima Flottiglia MAS crewmen came aboard, including LieutenantLuigi Durand de la Penne. On December 18 Sciré released the manned torpedoes 1.3 miles from Alexandria commercial harbor, and they entered the harbor when the British opened the boom defence to let three of their destroyers pass. After many difficulties, de la Penne and his crewmate Emilio Bianchi successfully attached a limpet mine under HMS Valiant, but had to surface as they attempted to leave and were captured. They refused to answer when questioned and were detained in a compartment aboard Valiant. Fifteen minutes before the explosion, de la Penne asked to speak to the Valiant's captain and informed him of the imminent explosion but refused to give other information. He was returned to the compartment and neither he or Bianchi were injured by the detonation of the mine. The other four torpedo-riders were also captured, but their mines sank the Valiant, the battleship Queen Elizabeth, the Norwegian tanker Sagona and badly damaged the destroyer HMS Jervis. The two battleships sank in only a few feet of water and were subsequently re-floated. Nevertheless they were out of action for over a year.
MG 120: MG 120 Shallow water attack submarine (SWATS) is a development of MG 110 in Pakistani service with the displacement is increased to 130 t submerged, range is 1600 nmi @ 7 kn surfaced, 60 nmi @ 4.5 kn submerged, and the number of divers carried is drastically increased to 15.
MG 130: MG 130 is a development of MG 120 with AIP module called UAPE (Underwater Auxiliary Propulsion Engine) based on a closed circuit diesel fueled by liquid oxygen, which provides an underwater range of 400 nmi (740 km).
Type B Kō-hyōteki-class Midget Ha 45 prototype built 1942 to test Type A improvements.
Type C Kō-hyōteki-class Midget Ha 62–76 similar to Type A with crew of 3 and radius increased to 350 nautical miles (650 km) at 6 knots (11 km/h) surfaced or 120 nautical miles (220 km) at 4 knots (7.4 km/h) submerged.
Type D Kō-hyōteki-class, also called Kōryū class (115 completed) improved Type C with crew of 5 and radius increased to 1,000 miles (1,600 km) at 8 knots (15 km/h) surfaced and 320 miles (510 km) at 16 knots (30 km/h) submerged.
S.X.756: Developed by Italian shipbuilder Cosmos as a successor of earlier S.X.506 midget submarine in Colombian service. Length is increased to 25.2 m and beam is increased to 2.1 m in comparison to its predecessor S.X.506, and displacement is increased to 73/80 t (surfaced/submerged). Crew is increased by one to a total of 6, but the total number of divers carried remained the same at 8. Range is 1600 nmi @ 6 kn surfaced, and 60 nmi @ 4 kn submerged, and payload remains the same.
MG 110 class midget submarine: Developed by Italian shipbuilder Cosmos as a successor of earlier S.X.756 midget submarine in South Korean service. MG 110 is a slightly enlarged version of S.X.756 with length increased to 27.28 m, diameter increased to 2.3 m, and displacement increased to 102/110 t (surfaced/submerged). Range is 1200 nm surfaced, and the submerged range remain the same. Payload and crew are also identical to that of its predecessor S.X.756 midget submarine.
Foca class submarine (design begun in 1957, entering service in 1962): A total of 2 units built, SA 41 & SA 42. Displacement is 16/20 t (surfaced/submerged), with a crew of 3, and armed with two 21 in torpedo tubes. Stricken in 1971. Not to be confused with World War II era Italian Foca-class submarine.
Tiburón class submarine (design begun in 1958, entering service in 1964): A total of 2 units built, SA 51 & SA 52. Displacement is 78/81 t (surfaced/submerged), with a crew of 5, and armed with two 21 in torpedo tubes. Stricken in 1971.
Spiggen II class midget submarine: 6 man crew midget sub displaces 14 t when submerged, with 5 kn speed and endurance of 2 weeks. Length: 11 m, beam: 1.4 m, height: 1.7 m.
Sea Dagger midget submarine developed by Kockums: a midget submarine of modular design with length of 16 – 20 m depending on various configurations totaling 4, including training, reconnaissance, diver delivery and small attack versions. All versions are consisted of 3 modules, two of which are identical for all versions, the bow and stern modules. The midsection has 4 modules to choose from, each for a specific mission. All versions lack the sail.
UVS-1300 Malen Originally built for navy purposes as sonar target. Midget submarine: 4 man crew midget sub displaces 11 t when submerged, with 5 kn speed and endurance of at least 1 week depending on fuel carried. Length: 10 m, beam: 1.4 m, height: 1.7 m.
Type 200: Displacement: 200 t surfaced, length: 25 m, beam: 4.2 m, draft 3.1 m, speed: 8 kn surfaced, range: 2100 nmi @ 8 kn surfaced, endurance: 2 weeks, armament: 2 21-in torpedo tubes with a maximum of 4 torpedoes, crew: 6 + 12 divers.
Type 300: Displacement: 300 t surfaced, length: 30 m, beam: 4.2 m, draft 3.1 m, speed: 12 kn surfaced, range: 3100 nmi @ 8 kn surfaced, endurance: 2 weeks, armament: 2 21-in torpedo tubes with a maximum of 6 torpedoes, crew: 12.
The Royal Navy has used a number of midget submarines. Most were developed during the Second World War. The decommissioning of the Stickleback class marked the end of midget submarines designed for combat in the Royal Navy.
X class: used to attack German warships in the north of Norway. One notable target of a successful attack was the battleshipTirpitz. The submarines had a crew of three (plus a diver) and carried two large mines containing Amatol – one on each side. Their strategy was to lay mines on the sea bottom beneath the target, set a time fuse, and leave.
Una-class midget submarine. After the Yugoslav Wars, both Croatia and FR Yugoslavia held examples. Length: 18 m, beam: 3.7 m, diameter: 2.5 m, displacement: 76 t surfaced, 90 t submerged, crew: 4 + 6 divers. Surfaced and submerged speed is 4 kn for both, and the range is 200 nmi surfaced, 50 nm submerged both @ 4 kn. M-100 lacks sail and its battery can only be charged when surfaced. Up to 4 R-1 swimmer delivery vehicle (SDV)s or two light (400 mm) torpedoes can be carried.
Type M-100D class midget submarine. Development of Type M-100 Una class. Displacement is increased to around 88/100 t (surfaced/submerged), and the length is increased to approximately 21 m, beam is increased to 4.42 m, and diameter to 2.7 m. Speed is increased to 8/11 kn (surfaced/submerged), and range is increased to 550 nmi surfaced @ 10 kn when surfaced, and that of submerged remains the same as its predecessor M-100 Una class while the payload and crew also remain unchanged. A sail is added and its battery can be charged while submerged.