In Ancient Greek theatre all actors wore masks, or prosopon in the ancient Greek, at all times on stage. These masks covered the entire face and much of the head. Each mask had a hairstyle, facial expression, and other distinct facial features, such as a beard. Because early Greek theatre usually only used two to three actors in each play, different masks allowed each actor to play more than one character. Masks were a carryover from deity worship, and were used to add a "mystical" value to the theatrical experience.

Masks were traditionally made from stiffened linen, carved wood, and/or tooled leather. Plain linen masks were introduced by Thespis, best known as the "first actor." Later, colorful and "terrifying" masks were used by Aeschylus, a tragic playwright. Another important breakthough was provided by Phrynichus, who first introduced the use of female masks.

Masks were used to allow actors, who were always male, to portray famous people, gods, animals, legendary figures, and women. Onomasticon, by Julius Pollox, listed the commonly used masks of the time. These were 28 for tragedy, and 44 for comedy (masks for satyrs were not listed). The most often used categories of masks were old men, young men, male servants, and women. Thus, as in later Roman theatre, stock characters became easily recognizable by the masks they wore. For example, the audience might learn over years of attending the Dionysus Festival that a certain angry male mask with a long beard was always an arrogant young male character, and could immediately assume as such upon seeing that mask used in a play.

Besides simply allowing for more characters and adding a mystic value to theatre, masks also helped to unify the chorus. For example, in Aristophanes' The Frogs, the chorus of frogs wore identical masks and costumes. Another purpose that many believe masks served was to amplify speech. However, this is most probably a myth, and new findings suggest that acoustics were so well tuned in Greek amphitheatres that no amplification was needed.