Metal Slug

The gameplay consists of run and gun elements as well as, in later releases, shoot 'em-up mechanics. The run and gun elements put a player against many enemies with the aid of extremely powerful weapon pick-ups. In most run and gun games, contact with an enemy leads to damage. In this series, however, contact results in the opportunity to perform a melee attack and the opportunity for the enemy to perform a melee attack of his own, if he has one.

The player's melee attack is also much stronger than most shots. This leads to the player's ability to run in and use melee attacks to take down a number of enemies at once, as well as the ability to quickly defeat enemies that can take plenty of damage, such as the mummies in Metal Slug 2.

The first game's story involved the Peregrine Falcon (PF) Squad, a small but skilled team of soldiers serving under the Regular Army's special operations division, who fight against the army of General Donald Morden in order to prevent a massive coup d'état and the creation of a New World Order. Later games featured characters from the Sparrows Unit, which is under the control of the Regular Army's intelligence division. Beginning with Metal Slug 2, the PF Squad also battles an alien threat to Earth (the Mars People), as well as several other supernatural threats including yetis, zombies, ambulatory Venus flytraps, giant crabs, and mummies. These outlandish elements were removed from the fifth game to return to the feel of the original title.

The fifth moved to the motif of modern guerrilla warfare, leaving only traces of the series' signature quirky humor and paranormal enemies, with the exception of the final boss. Metal Slug 6 returned to the plot of the first three installments, bringing back Morden's Rebel Army and the Mars People. Metal Slug 7 has less outlandish elements with the Mars People replaced by an alternative universe of Morden's Army with futuristic equipment and weapons.

The aspect of the Metal Slug series was to create a simple, but exciting side-scrolling shoot-em-up game with an easy control scheme (one joystick and three buttons). The same team that created Metal Slug for the Neo-Geo previously created several games for Irem which have very similar graphics and gameplays, such as GunForce (1991), In the Hunt (1993) and Gunforce 2 (1994).

Some of the stock sound effects and sprites from Irem and The King of Fighters titles were used in the Metal Slug games. The music was composed by Takushi Hiyamuta (who is credited as Hiya!), a key member of the Nazca Corporation, which was acquired by SNK in 1996. The first three titles were developed by the Nazca team before SNK declared bankruptcy in October 2001, leading to the disbandment of the original team. Anticipating the collapse of SNK, the company Playmore was launched in August 2001, which became known as SNK Playmore in 2003, the year after Metal Slug 4 was released. Development on the series has since continued without the original staff, with Noise Factory handling 4, 5, 6, and 7.