Quarterback (originally called blocking back) is a position in American and Canadian football. Quarterbacks are members of the offensive team and line up directly behind the center, in the middle of the offensive line. Quarterbacks are the leaders of the offensive team, responsible for calling the play in the huddle. They initiate almost all plays by receiving the ball from the center, although occasionally teams will use a 'direct snap' (usually as a trick play) to another offensive player. Once the quarterback receives the snap, he can run with the ball, hand the ball to another player or attempt a forward pass.

At most levels, but especially at the college and professional level, the quarterback role is one of the most visible and important roles on the team. The quarterback touches the ball on nearly every offensive play and has a great deal of responsibility both in calling plays and making decisions during the play. While there is liberal substitution at most positions in football based on the play call and to minimize player fatigue, most quarterbacks are on the field for every offensive play leaving only for injury or when the game's outcome is no longer in doubt. Quarterbacks are frequently chosen early in the NFL Draft and often receive much more lucrative contracts than other positions. As of 2008, players in this position have won more Super Bowl MVP awards (22 of 43) than players at any other position combined.

As the term "quarterback" gained acceptance in the 1930s, it originally referred to the player's position relative to other members of the offensive backfield. Before the emergence of the T-formation in the 1940s, all members of the offensive backfield were legitimate threats to run or pass the ball, and most teams used four offensive backs on every play: a quarterback, two halfbacks, and a fullback. The quarterback began each play a quarter of the way back, the halfbacks began each play side by side and halfway back, and the fullback began each play the farthest back. Now that most offensive formations have only one or two running backs, the original designations don't mean as much, as the fullback is now usually a lead blocker (technically a halfback), while the halfback or tailback (called such because he stands at the "tail" of the I) lines up behind the fullback.

Special tactics

If quarterbacks are uncomfortable with the formation the defense is using, they may call an audible to change their play. For example, if a quarterback receives the call to execute a running play, but he notices that the defense is ready to blitz, the quarterback may want to change the play. In order to do this, the quarterback will yell a special code, like “Blue 42” or “Texas 29”, which informs the offense to switch to a specific play or formation.

Also, quarterbacks can "spike," or throw the football at the ground, in order to stop the official game clock. For example, if a team is down by a field goal with only seconds remaining, a quarterback may spike the ball in order to prevent the game clock from running out. This usually allows the field goal unit to come onto the field, or attempt a final "Hail Mary pass". However, if a team is winning, a quarterback can keep the clock running by kneeling after the snap. This is normally done when the opposing team has no timeouts and there is little time left in the game, as it allows a team to burn up the remaining time on the clock without risking a turnover or injury.