Realism (also known as Tier 1) is a multiplayer game type introduced in Call of Duty: Modern Warfare. It would return in Call of Duty: Modern Warfare II as Tier 1 added as part of the Season One update before being removed and replaced with Hardcore with the Season Two update.
In Realism, the player has a limited HUD, low health, slow health regeneration and is able to kill friendly players. The mode is playable on all Maps and modes featured in Modern Warfare and Modern Warfare II.