The MP5 was first introduced by Heckler & Koch in 1966, under the name HK54. This name comes from HK's old numbering system: the "5" designates the model as a submachine gun, while the "4" identifies it as being chambered for 9 × 19 mm ammunition. The current name dates from when it was officially adopted by the West German government for use by its police and border guard as the Maschinenpistole 5 ("Machine pistol 5", or MP5), in mid-1966. The GSG 9 (the counter-terrorist unit of the German Federal Police) then introduced the MP5 to other Western counter-terrorist units.

With the increased use of body armor, the future of the MP5 is uncertain. Several new trends in firearms design have begun to eclipse the submachine gun; small caliber personal defense weapons (PDW) like the new Heckler & Koch MP7 and compact carbines such as the M4, AKS-74U, the G36C variant of HK's G36. The only major criticism of the MP5 has been its high cost—approximately 900 USD for an MP5N (the United States Navy variant).[citation needed] Heckler & Koch has started to complement the MP5 series with the cheaper UMP, which is available in .45 ACP, .40 S&W and 9 mm Parabellum calibers. However, since the UMP uses a simple blowback action as opposed to the MP5's roller-delayed blowback, the two weapons may not necessarily be competitors among the most discriminating users. In addition, the lighter weight of the UMP makes it more difficult to control during fully automatic fire than the MP5.

One famous counter-terrorist operation involving the MP5 was Operation Nimrod. It took place on April 30, 1980 in the United Kingdom, when the Special Air Service (SAS), armed with MP5s, was deployed to assault the terrorists who had taken over the Iranian embassy in London.

The MP5's accuracy, reliability, and wide range of accessories and variations have made it the submachine gun of choice for military and law enforcement agencies worldwide for over thirty years. Users include counter-terrorist groups, special operations forces and police forces.

[edit] Design
A typical MP5 fires 9 × 19 mm Parabellum ammunition from a curved, detachable box magazine; magazine capacity is 15 or 30 rounds.

As with most weapons produced by Heckler & Koch, the MP5's trigger group is a replaceable modular component. The trigger group has a fire mode selector, which may include various combinations of fully automatic, 3- or 2-round burst, and semi-automatic, in addition to a safe position.

MP5s were originally chambered for the 9 mm Parabellum cartridge, commonly used in pistols. In the late 1990s, more powerful variants chambered for the 10 mm Auto and .40 S&W cartridges (MP5/10 and MP5/40, respectively) were introduced. In 2000, however, both versions were discontinued in favor of the new UMP submachine gun.

The MP5 has a roller-delayed blowback mechanism tracing back to a mid-1940s German prototype known as the StG45(M) and further developed in the CETME rifle.

Heckler & Koch has made many general improvements to the MP5 over the years since its introduction; both internal and external components changed considerably from the earlier models to the present design. The original MP5 was available with a fixed (A2) or retractable (A3) stock, and in various trigger group configurations.

The next major development was the MP5SD series (SD1–SD6), introduced in 1974. This model had an integrated suppressor and a special barrel which reduced the muzzle velocity of its ammunition to just below the speed of sound. The result was that the MP5SD series is almost inaudible at distances of more than 15 meters. By its remarkable sound and almost invisible muzzle flash, it is chosen as a stealthy-operating weapon in many police and military forces.

The MP5K ("K" standing for kurz, meaning "short"), which is only 325 mm long, was introduced in 1976 at the request of a HK South American sales representative who saw the potential for its sale to bodyguards as a concealable, but fully automatic weapon. It has a vertical foregrip to reduce muzzle rise and aid in automatic firing and it can also be fired from inside a special briefcase. Additionally, a version without the diopter sights but a smooth upper surface and small pistol sights was developed to reduce the likelihood that the weapon would snag during the draw.

A further development of the model by the U.S. division of HK was the MP5K-PDW (Personal Defense Weapon), in 1991. This model was built for United States Air Force pilots who needed a compact weapon. Unlike the original, the PDW adds a folding stock and can accept a suppressor and laser sight. All variants of the MP5K are available in the similar configurations as the original weapon.

Semi-automatic versions of the MP5 and MP5K, known as the HK 94 and SP89 respectively, were offered prior to the 1994 Assault Weapons Ban. Some of these semi-automatic models have been legally converted to submachinegun form by either modifying the lower receiver to take a select-fire trigger mechanism or by modifying the trigger mechanism of the semi-automatic lower receiver. A drop-in/registered (transferable) auto-sear is also frequently utilized.

Under a special contract from the United States Navy, Heckler & Koch developed the MP5N ("N" standing for "Navy") variant for the Navy's elite special operations units (including the U.S. Navy SEALs). The MP5N is a standard MP5 with a fully ambidextrous Navy trigger group (with safe, semi- and fully automatic modes), a retractable stock, and a threaded barrel for mounting stainless steel suppressors.

The latest in the MP5 line is the MP5F ("F" standing for French), developed in 1999 specifically for the French military. The MP5F is an MP5 with some new internal parts modifications design to handle extremely powerful 9 mm ammunition. The changes to the MP5F are subtle. Externally, the only noticeable differences between the "F" model and standard MP5s are a new buttstock that has a two-inch thick rubber buttpad, a sling loop on the right side of the stock for left-handed shooters and dual front sling bolts.

The MP5/10, chambered in the 10 mm Auto cartridge, was Heckler & Koch's first attempt to increase the power of the MP5 series. In 1994, the U.S. Federal Bureau of Investigation (FBI) adopted the MP5/10 as their standard submachine gun. Despite the discontinuance of MP5/10 production and the agency's adoption of the .40 S&W cartridge for pistols, the FBI recently solicited HK for the purchase of an additional fifty MP5/10. The name MP10 has been incorrectly used by some to indicate the MP5/10. This error was popularized by Tom Clancy's book Rainbow Six.