In contrast to members of the offense, the rules of professional football (NFL Rulebook) and American college football (NCAA Rulebook) do not specify starting position, movement, or coverage zones for members of the defensive team, except that they must be in the defensive zone at the start of play. The positions, movements and responsibilities of all defensive players are assigned by the team by selection of certain coverages, or patterns of placement and assignment of responsibilities. The positional roles are customary. These roles have varied over the history of American football. The following are customary defensive positions used in many coverages in modern American football.
- The defensive line consists of three to six players who line up immediately across from the offensive line. They try to occupy the offensive linemen in order to free up the linebackers, disrupt the backfield (behind the offensive line) of the offense, and tackle the running back if he has the ball before he can gain yardage or the quarterback before he can throw or pass the ball. They are the first line of defense.
- Behind the defensive line are the linebackers. They line up between the defensive line and defensive backs and may either rush the quarterback or cover potential receivers.
- The last line of defense is known as the secondary, comprising at least three players who line up as defensive backs, who are either cornerbacks or positionsafeties. They cover the receivers and try to stop pass completions. They occasionally rush the quarterback.