With some exceptions, the games belong to the Run 'n' gun genre—sidescroller platform games with Shoot em' up elements, where the player must run through a stage while shooting at various enemies. The first game was released in 1996 for the Neo Geo MVS arcade platform.
In the first game, the players take the role of the Peregrine Falcon Squad soldiers Marco Rossi and Tarma Roving, and must face the villain General Morden and his Rebel Army, who wants to eliminate the governments and establish a new order, all of this fueled by his anger caused by the death of his son. Later on the series, Marco and Tarma are joined by S.P.A.R.R.O.W.S agents Eri Kasamoto and Fio Germi, as Morden makes an allegiance with an alien race and other military groups.
The game uses a standard 3-button configuration: a shot button, a jump button and a grenade button. The player starts the stages with a simple handgun with infinite ammo and 10 grenades. Through the missions the player can find several weapons which are more powerful but have limited ammo, as well as ammo and grenades. During a mission, the player must destroy the enemies and their machinery, while rescuing the POW's who are scattered in the missions. When rescued, the POW's will reward the player with weapons, ammo or elusive items which gave bonus points for the player. Each POW rescued is counted at the end of a stage, giving extra points to the player. However, if the player dies during a mission, the POW count will be reset to zero. Many POW's and items are hidden in the missions, and they can be found by shooting at specific points of the scenery. The player will also be able to use vehicles know as Slugs, which are equipped with a Vulcan with unlimited ammo and a cannon, and protects the player for some hits. The player an leave a slug anytime by pressing down + jump, as well as sending the vehicle to explode at the enemies by pressing shot + jump. If the vehicle takes many hits, it will start flashing red, and the player must evacuate it before it explodes.
Metal Slug 6 introduces a new weapon-stock system similar to Contra, where the player can carry more than one weapon at once, being able to switch between them by using a button, being also able to discard a weapon. Also in the same game, the characters are given special skills, such as carrying more ammo or grenades or performing special attacks by using a button combination.
The main series of Metal Slug was created mainly for the SNK's Neo-Geo system. In 2006, SNK Playmore dropped the now 15-year old Neo Geo in favor of Sammy's Atomiswave arcade platform. In 2008, the series was taken to the Nintendo DS handheld platform. Many spin-offs have been created for other systems.
- Metal Slug: Super Vehicle-001 (1996) - Neo Geo, Neo Geo CD, Sega Saturn, PlayStation
- Metal Slug 2 (1998) – Neo Geo, Neo Geo CD
- Metal Slug X (1999) – Neo Geo, PlayStation
- Metal Slug 3 (2000) – Neo Geo, PlayStation 2, Xbox
- Metal Slug 4 (2002) – Neo Geo, PlayStation 2, Xbox, GameCube
- Metal Slug 5 (2003) – Neo Geo, PlayStation 2, Xbox, GameCube
- Metal Slug 6 (2006) – Atomiswave, PlayStation 2
- Metal Slug 7 (2008) – Nintendo DS
- Metal Slug XX (2009) – PlayStation Portable
- Metal Slug: 1st Mission (1999) – Neo Geo Pocket Color
- Metal Slug: 2nd Mission (1999) – Neo Geo Pocket Color
- Metal Slug Advance (2003) – Game Boy Advance
- Metal Slug 3D (2006) – PlayStation 2