Teacher unions (or alternatively “education unions”) are organizations formed to protect and advance the collective interests of teachers and other education workers. What the collective interests of educators entail and how they should be pursued have been and remain active matters for debate within these organizations. Different unions at different times have responded differently to these questions, for example, in relation to the degree to which an industrial versus a professional orientation should be adopted, and the degree to which a wider political and social justice agenda should be embraced. Several ideal-type models of teacher unionism have been identified, as well as various strategic options that these unions might employ. A spirited debate is ongoing about the legitimacy and power of teacher unions. One perspective portrays them as self-interested special interest groups, and another as social movements advocating for public education. The status of teacher unions as stakeholders in educational policymaking is contested, and union–government relations occur across a spectrum of arrangements ranging from those that encourage negotiation to those characterized by confrontation and hostility. Internationally, education unions face significant challenges in the early decades of the 21st century. Neoliberal economic and industrial policies and legislation have eroded the capacity of unions to collectively organize and bargain, and the global education reform movement (GERM) has created a hostile environment for education unions and their members. Despite these challenges, education unions remain among the most important critics of GERM and of global neoliberal social policy generally. The challenges posed and the strategies adopted play out differently across the globe. There is evidence that at least some unions are now prepared to be far more flexible in adopting a “tapestry” of strategies, to examine their internal organization, build alliances, and develop alternative conceptions of the future of education. Researchers, however, have identified certain internal factors in many teacher unions that pose significant obstacles to these tasks. Unions face difficult choices that could lead to marginalization on the one hand or incorporation on the other.