An understudy is a performer who learns the lines, blocking, and choreography of a regular actor, actress, or other performer in a play, referred to in opera as cover or covering. The understudy is responsible for being able to take over the part should the regular actor or actress be unable to appear on stage. The theater manager usually announces the cast change prior to the start of the performance. The term understudy was coined in 1874 and has more recently been applied only to performers who can back up a role but still regularly perform in another role.

In some instances, multiple understudies are assigned to cover the same role and ranked by priority. If the principal actor is unable to perform, the role devolves to the highest-priority understudy who is available. Unlike swings, an understudy is in fact in the cast of a show for every performance, generally in an ensemble track or less sizable principal role. However, they also understudy a larger role, and their track will be filled by a swing if and when they are called upon to play that larger part.

Understudies are essential in ensuring that the show is always ready for performance in the event of an unexpected injury, illness, or if a principal is taking a vacation. Accepting an understudy role is a tremendous responsibility and one that should not be taken lightly. Time spent should focus on perfecting knowledge of the role.