Ancient Greek Theater
The earliest days of theater remain obscure, but the oldest surviving plays come from ancient Greece. (Is this true? What about China?)
Dramatic and theatrical productions in ancient Greece may have come from the revelry of the Bacchus, an ancient Greek festival in honor of the god Dionysos. In any case, the modern theater was born in ancient Greece. Important playwrights in ancient Greek theater include:
Aristotle is also important, primarily for his timeless theories on the dramatic arts. Please note that his theories, especially the Three Unities (I don't know if this is the correct term) have been disputed. Some scholars believe they are meant to be descriptive, not prescriptive.
The above-mentioned playwrights made some of the most renowned Greek plays, but their staging had little or nothing to do with twentieth-century theater. Their dramas were always part of a series of three performances, where the middle part only was the drama, while the events always ended with dance. The dramas rarely had more than three actors (all male), who played the different roles using masks. There was a chorus on the stage all the time which sang songs and sometimes spoke in unison. As far as we know, each drama was played just a single time, at the traditional drama contest.
The importance of ancient Greek theater came largely in retrospect, as major playwrights like Johann Wolfgang von Goethe tried to recreate classical theater unsuccessfully. Another attempt to revive classical theater argued that Greek actors didn't speak, but sang. From this school came the opera.
Please add other historical theaters: Chinese, Japanese (Kibuki?), other Asian, etc., etc. Let's not be too English-centric.
Theater in the Middle Ages
In the Middle Ages in Europe, theater was a vital part of people's civic, economic, and religious lives. Among the more notable religious plays were "The Summoning of Everyman" (an allegory designed to teach the faithful that acts of Christian charity are necessary for entry into heaven), passion plays (such as the one at Oberammergau, which is still performed every 10 years), and the great cycle plays (massive, festive wagon-mounted processions involving hundreds of actors, and drawing pilgrims, tourists, and entrepreneurs) 1.
In an age when religion influenced nearly all aspects of public and private life, there was little formal audience for secular theater; nevertheless, wandering mistrels and folk plays developed even as the religious pageants expanded.
Since many of the most theatrically successful medieval religious plays were designed to teach Catholic doctrine, the Protestant Reformation targeted the theater, especially in England, in an effort to stamp out allegiance to Rome. See Eamon Duffy's The Stripping of the Altars: Traditional Religion in England, c.1400-c.1580 (1994).
Commedia del Arte
Rise and influences only, please. Modern theater should mostly go under theater, directly.