Actors' Equity Association (AEA) -Labor union representing American actors and stage managers in the theater.
American Society for Theatre Research (ASTR) - Under Resources/Directories are links to theater organizations, libraries and archives, primary and secondary sources and publishers.
Association for Theatre in Higher Education (ATHE) - Click on "Research" for an excellent list of theater specific journals, links to organizations, conferences and funding opportunities.
Broadway League - National trade association for the commercial theater industry. Site provides a wide variety of statistics, stage specs guides, and specs for Broadway venues.
Dramatists Guild of America - Professional organization of American playwrights, lyricists and composers.
International Theatre Institute Sponsored by UNESCO, this site Includes complete lists of productions, a world theater directory, and links to international theater and technical theater organizations.
League of Resident Theaters (LORT) - Professional association of American nonprofit theaters, including links to internship application forms.
Literary Managers and Dramaturgs of the Americas (LMDA) - North American network of dramaturgs includes links to relevant blogs and discussion lists, as well as a handbook for student dramaturgs.
Society of Dance History Scholars (SDHS) - Organization dedicated to Western and non-Western dance history. Interesting links.
Stage Directors and Choreographers Society (SDC) - Professional theatrical union, encompassing a broad sepctrum of venues.
Theatre Communications Group (TCG) - Organization dedicated to the promotion of nont for profit theater in the United States. Great practical resources.
Theatre Library Association (TLA) - Valuable for lists of performing arts libraries, archives and museums, as well as for links to theater resources.
United Scenic Artists - A labor union and professional association of designers, arts and craftspeople.
United States Institute of Technology (USITT) - Wonderful networking site.