This mixed-method case study explored teacher union members' beliefs about the teacher union and their reasons for being active or inactive in the union. Findings suggest that teacher unions have gained pragmatic and cognitive legitimacy (Chaison and Bigelow in Unions and legitimacy. Cornell University Press, Ithaca, NY, 2002), but that participants' perceptions of the union's moral legitimacy (Chaison and Bigelow in Unions and legitimacy. Cornell University Press, Ithaca, NY, 2002) influenced their beliefs about the union. Specifically, participants' beliefs about the union's role in job protection, especially the protection of ineffective teachers, and social-professional supports (or lack of) strongly influenced their decisions to be active or inactive in the teacher union. These findings have implications for how effectively teacher unions are able to engage current members and sustain member engagement in the future.