Call of Duty 4: Modern Warfare is a 2007 first-person shooter game developed by Infinity Ward and published by Activision. It is the fourth main installment in the Call of Duty series. The game breaks away from the World War II setting of previous entries and is instead set in modern times. Developed over two years, Modern Warfare was released in November 2007 for the PlayStation 3, Xbox 360, and Microsoft Windows. A Wii port, developed by Treyarch and subtitled Reflex Edition, was released in 2009. It was followed by two sequels that continue the storyline: Modern Warfare 2 (2009) and Modern Warfare 3 (2011).

The game was praised by critics, especially for its gameplay and storyline, winning numerous "Game of the Year" titles and two BAFTA Video Games Awards. It was the top-selling game worldwide in 2007, selling around seven million copies by January 2008 and almost sixteen million by November 2013. Retrospective reviewers consider it one of the best, if not the best, games in the series, and one of the greatest video games ever made for its influential campaign and multiplayer. The multiplayer portion of the game features numerous game modes and contains a leveling system that allows the player to unlock additional weapons, weapon attachments, and camouflage schemes as they advance.

A remastered version of the game, developed by Raven Software and titled Call of Duty: Modern Warfare Remastered, was released as part of special edition bundles of Call of Duty: Infinite Warfare in November 2016 and as a standalone game in June 2017. A reboot of the Modern Warfare game, Call of Duty: Modern Warfare, was released in October 2019.

Call of Duty 4: Modern Warfare is a first-person shooter. A character can be positioned in one of three stances: standing, kneeling, 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 there is no armor or health powerups. 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.^{[citation needed]}

The player takes on the role of various characters during a single-player campaign. The characters' involvement in the plot occurs simultaneously and overlaps the events in the game. As such, the player's perspective changes from one character to another between missions. Each mission features a series of objectives; the player is led to each objective with the heads-up display, which marks its direction and distance. Some objectives require that the player arrives at a checkpoint, while other objectives require the player to eliminate enemies in a specified location, stand their ground to defend an objective, or plant explosive charges on an enemy installation. After completing the campaign, a special epilogue mission "Mile High Club" is unlocked for play. The mission itself has no bearing on the campaign plot and focuses on a SAS squad fighting terrorists that have hijacked an airplane and taken a VIP hostage. The main campaign features 30 collectible pieces of intel that award the player with game cheats and visual filters such as infinite ammunition, cluster grenades, and increased contrast.

Call of Duty 4: Modern Warfare features team-based and deathmatch-based multiplayer modes on various maps. Each mode has an objective that requires unique strategies to complete. Players can call in UAV reconnaissance scans, airstrikes, and attack helicopters when they achieve three-, five-, and seven-enemy kill streaks respectively. A game ends when either a team or player has reached a predefined number of points, or the allotted time expires in which case the team or player with the most points wins. If the points are even when the time expires, Sudden Death mode is activated in which there is no re-spawning, and the team that either has the last man standing or achieves the objective first is the winner. If the player is in either of the two matches, then there is an Overtime match, in which the next team to win is rewarded the victory.

The player's performance in the multiplayer mode is tracked with experience points, which can be earned by killing opposing players, completing challenges, completing objectives, or by completing a round or match. As the player gains experience, they advance in level, unlocking new weapons, perks, challenges, and gameplay modes. The highest obtainable level is 55, but on the console versions of the game, the player has the option to enter "Prestige" mode, which returns their level to one and removes all accumulated unlockables. This process can be repeated up to 10 times with a different insignia being given each time.