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 Regular Army's 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 family that could have been prevented if the Regular Army was not corrupt. 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 multiple allegiances with an alien race and other military groups.
The game uses a standard 3-button configuration: a shoot button, a jump button and a grenade button. The player starts each mission with a simple handgun with infinite ammo and 10 grenades. Throughout the missions the player can find several weapons which are more powerful but have limited ammo, as well as ammo and grenades. At the end of a mission lies a large boss to fight.
During a mission, the player must destroy the enemies and their machinery, while rescuing the POWs who are scattered in the missions. When rescued, the POWs 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 POWs 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 known as Slugs, most of 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 shoot + jump. If the vehicle takes many hits, it will start flashing red, and the player must evacuate it before it explodes.
By default settings, a player starts a mission with three lives and cannot earn more. A player who loses all of their lives can continue by inserting more credits before time runs out. If they continue, then they select their character and pick up a dropped Heavy Machine Gun. After the game is over, a player who hit the games' high-score table can insert their 3-letter initial to the Best Tank Busters.
Later on the series, new movements and commands are added to the game. In Metal Slug 4, the Metal Slug Attack command has been put as its own separate button. In Metal Slug 5, the player can use a Mega Man-like sliding move by pressing Down + Jump; this move was excluded in the sequels.
- 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, and also able to discard a weapon.
- A revamped combo system where players are given multiplied points for destruction they ensue. When enough damage is given, coins worth an exponential amount of points pop out of enemies.
- Each of the playable characters are given their own unique special skills, such as carrying more ammo or grenades or performing special attacks by pressing 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 and mobile devices.
- Metal Slug: Super Vehicle-001 (1996) - Neo Geo, Neo Geo CD, Sega Saturn, PlayStation, Steam
- Metal Slug 2: Super Vehicle-001/II (1998) – Neo Geo, Neo Geo CD, Steam
- Metal Slug X: Super Vehicle-001 (1999) – Neo Geo, PlayStation, Steam
- Metal Slug 3 (2000) – Neo Geo, PlayStation 2, Xbox, Steam
- 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, Xbox 360, PlayStation 4, Steam
- Metal Slug: 1st Mission (1999) – Neo Geo Pocket Color
- Metal Slug: 2nd Mission (2000) – Neo Geo Pocket Color
- Metal Slug Advance (2003) – Game Boy Advance
- Metal Slug 3D (2006) – PlayStation 2
- Metal Slug Tactics (TBA) - Steam
Modern Smartphone Spin-Offs:
- Metal Slug Defense (2014) - Android, iOS, Steam
- Metal Slug Attack (2016) - Android, iOS
- Metal Slug: Commander (2021) - Android, iOS
- Metal Slug: Awakening (2022) - Android, iOS