The freedom of association designates a special role for trade unions in voicing the interests of workers and participating in the governance of the labour market. This chapter argues that trade unions are distinguished from other institutions of workers’ voice by their membership basis. However, the unity of trade unions conceals two distinct logics of what trade unions do, that is, enterprise-based and social-wide bargaining. The chapter distinguishes between the two on basis of their relationship with the fundamental justifications for the freedom of association, that is, democracy, equality, dignity and solidarity, and the ancillary bases for trade unions' actions – accountability, legitimacy and power. From these differences stem inherent tensions as well as the potential for complementarity, which will frame the study of membership-based strategies for trade unions’ revitalization efforts in the following chapters.