This paper reviews and up-dates previous work on the extent of compulsory unionism, or the closed shop, in Australia. It uses data from a secondary analysis of the Australian Workplace Industrial Relations Survey to estimate the extent, factors and sources of the closed shop in Australia. Four main conclusions are reached. First, the closed shop is a widespread phenomenon. Second, the main sources of variation in the closed shop are mainly sectoral, occupational and industrial. Third, the main sources of the closed shop are union-management agreements, custom and practice, and preference provisions in awards. Fourth, it is tentatively concluded that stability has characterized the closed shop at an aggregate level over the last decade, with changes being specific to certain industries.