Call of Duty 4: Modern Warfare is a first person shooter. A character can be positioned in one of three stances: standing, crouching, or prone, each affecting the character's rate of movement, accuracy, and stealth. Using cover helps the player avoid enemy fire or recover health after taking significant damage. As such, there are no armor or health power-ups. When the character has taken damage, the edges of the screen glow red and the character's heartbeat increases. If the character stays out of fire, the character can recover. When the character is within the blast radius of a live grenade, a marker indicates the direction of the grenade, helping the player to either flee or toss it back to the enemy.
The game is the first in the Call of Duty series to feature modern equipment. The game also introduces new features, particularly multiplayer mechanics such as "killstreaks" where the player gains access to special abilities for killing enemies without dying.