[12] Featuring a compact bullpup design with an integrated reflex sight and fully ambidextrous controls, the P90 is an unconventional weapon with a futuristic appearance.

The P90 is currently in service with military and police forces in over 40 nations, such as Austria, Brazil, Canada, France, Greece, India, Malaysia, Poland, and the United States.

[14] In the United States, the standard selective fire P90 is restricted to the military, law enforcement, or holders of certain Federal Firearms Licenses (FFLs) with the Special Occupational Tax (SOT).

[15] The P90 and its 5.7×28mm ammunition were developed by the Belgian company FN Herstal in response to NATO requests for a replacement for the 9×19mm Parabellum cartridge and associated pistols and submachine guns.

"[16] In 1989, NATO published document D/296, outlining a number of preliminary specifications for these weapons: FN Herstal was the first small arms manufacturer to respond to NATO's requirement; FN started by developing a shoulder-fired personal defense weapon, the P90, along with a small caliber, high velocity 5.7×28mm cartridge type.

[18] The results of the NATO tests were analyzed by a group formed of experts from Canada, France, the United Kingdom, and the United States, and the group's conclusion was that the 5.7×28mm was "undoubtedly" the more efficient cartridge;[18] However, the German delegation and others rejected the NATO recommendation that 5.7×28mm be standardized, halting the standardization process indefinitely.

[17] The P90 fires from a closed bolt for maximum accuracy, and its design makes extensive use of polymers for reduced weight and cost.

[17] The P90 was designed to have a length no greater than a man's shoulder width, to allow it to be easily carried and maneuvered in tight spaces, such as the inside of an armored vehicle.

[10] To achieve this, the weapon's design uses the unconventional bullpup configuration, in which the action and magazine are located behind the trigger and alongside the shooter's face so that there is no wasted space in the stock.

[15] The P90's dimensions are minimized by its unique horizontally mounted feeding system, wherein the box magazine sits parallel to the barrel on top of the weapon's frame.

[8] The barrel has eight rifling grooves with a right-hand twist rate of 1:231 mm (1:9.1 in), and it is equipped with a diagonally cut flash suppressor that also acts as a recoil compensator.

[10][23] One of the design intents for the standard 5.7×28mm cartridge type, the SS190, was that it has the ability to penetrate Kevlar protective vests that stop conventional pistol bullets.

[7] In testing conducted by the Royal Canadian Mounted Police (RCMP) in 1999, the SS190 fired from the P90 at a distance of 25 m (27 yd) exhibited an average penetration depth of 25 cm (9.8 in) in ballistic gelatin covered with a Level II vest.

[27] The SS190 exhibited penetration depths ranging from 28 to 34 cm (11 to 13.5 in) when fired from the P90 into bare ballistic gelatin, in tests conducted by Houston Police Department SWAT.

[10] Since the SS190 projectile does not rely on fragmentation or the expansion of a hollow point, the cartridge (and 5.7×28mm weapons) is considered suitable for military use under the Hague Convention of 1899, which prohibits the use of expanding bullets in warfare.

[29] The P90 uses a unique horizontally mounted feeding system—patented in the United States—that contributes to the weapon's compact profile and unusual appearance.

[33] The detachable box magazine is mounted parallel to the P90's barrel, fitting flush with the top of the weapon's frame, and it contains 50 rounds of ammunition which lie in two rows facing left, offset 90° from the bore axis.

[17] As the cartridges are pushed back by spring pressure and arrive at the rear end of the magazine, they are fed as a single row into a spiral feed ramp and rotated 90 degrees, aligning them with the chamber.

The night reticle for both the HC-14-62 and the MC-10-80 consists of an open T that is primarily illuminated by a tritium module, and, in the HC-14-62, ambient light drawn in by an upward-facing collector.

[citation needed] The P90 has provisions for mounting a number of different types of accessories, including tactical lights and laser aiming devices.

A sling can be attached to the P90 for greater ease of carry, or it can be fitted with various sound suppressors such as the Gemtech SP90, which was designed specifically for the weapon in cooperation with FN Herstal.

[7] This stainless steel suppressor with a black oxide finish is built according to MIL-SPECs, including saltwater corrosion resistance.

[34] The black reticle consists of a tiny dot inside of a small ring, which is joined by three posts that glow red in low light conditions due to tritium-illumination.

[8][37][self-published source] Both laser systems are compact, consisting of a small, flat panel integrated into the front end of the weapon's frame.

[37] The Laserex P90 laser systems have a weight of 131 grams (0.289 lb; 4.6 oz), and they are activated by means of a green pressure switch located on the underside of the weapon's pistol grip.

[37] The PS90 is a semi-automatic carbine variant of the P90, intended for civilian shooters for personal protection and sporting use; it was introduced in 2005, and continues to be offered in several configurations.

[21] It features a MIL-STD-1913 (Picatinny) rail fitted to the top of the receiver, which allows the shooter to mount their preferred optical sight.

[34] The black reticle consisted of a tiny dot inside of a small ring, which was joined by three posts that glowed red in low light conditions due to tritium-illumination.

[13] In the United States, Houston Police Department was the first local law enforcement agency to adopt the P90, acquiring it for their SWAT team in 1999.