In American football, a spy is a defensive player assigned to cover the opposing team's quarterback man-to-man.

This strategy is generally used against "dual-threat" quarterbacks who are capable of rushing when passing plays break down. The defensive player floats near the line of scrimmage, following the quarterback's movements.^[1]

Spies are also sometimes known as "keys".^[2] Generally, spies are linebackers.^[3]