The main fighting force in Bowser's army, Koopa Troopas are one of Mario's oldest foes. First appearing as "Shellcreepers" in the arcade title Mario Bros., Koopas have appeared in almost every Mario game since, although their behavior has significantly changed through time. SMB3 saw the first instance of Mario being able to carry the turtles' shells, and SMW took this concept a step further, allowing Koopas to leave their shells completely, continuing to pose a danger to Mario, even after being stomped.

Although Koopa Troopas have usually been enemies of the Mario Brothers, they have recently taken a change of heart and now often appear as friendly characters. Some of the more prominent friendly Koopas have appeared in Mario Kart, SM64, and Paper Mario.