Sibmas, the International Association of Libraries and Museums of the Performing Arts. With links to the International Directory of Performing Arts Collections and Institutions, where you can find information such as email addresses, phone numbers, hours of operation, etc., for most of the world's performing arts collections.
Gesellschaft für Theatergeschichte,
Society for Theatre Geschichte
Thalia Germanica International society for research into the history of the German-language theatre abroad.