The Greek poet Thespis -- the so-called "father of the drama" -- was the first to use a mask in ancient Greek theatre. In fact, "thespian," another word for actor, is derived from his name. Masks that the actors wore in Greek theatre were made of linen, wood or leather. The masks played a critical role for many reasons, enabling actors to portray multiple roles and to display various emotions of the characters.
In ancient Greek theatre, men played all roles, because at the time women were not allowed on stage. Males who had not reached puberty and whose voices had not changed even portrayed women and girls. Masks along with costume pieces such as prosthetic breasts and prosthetic bellies would distinguish the female characters.
Greek theatre was notable for having few actors in a play, so each actor played multiple roles. Masks would be made to represent each character in the play. The actors could quickly change the mask to assume the role of each character when needed.
In Greek theatre, masks could be very large and displayed a wide variety of emotions. The mask would convey to the audience to if the character was happy, sad, tired or scared, even if audience members were sitting in the back row of the theatre.
It is also theorised that masks used in Greek theatre could aid in the amplification of the actors' voices so the audience could hear them better. However, some debate whether the mask actually provided this feature. The theatres of ancient Greece had very sound acoustics allowing the audience to hear even whispers.
Even the chorus of the ancient Greek theatre wore masks during performances. Their masks were very similar to those of the actors on stage but were less ornate and expressive.