The freedom of association in trade unions is accorded a special status in the international covenants on human rights, in ILO Conventions and in national constitutions. Moreover, trade unions are usually the sole subjects of special privileges for negotiating collective agreements, consultation and permissible industrial action. Human rights and constitutional litigation usually focus on removing obstacles from individuals’ choice to associate and on the scope of the positive right to freely associate. However, there are other fundamental aspects of the right that are more covert, which are designated as its ‘hidden components’. These include the rules regarding representative status, which crudely prescribe the relationship between trade unions and their membership. Despite the prevalence of requirements for representativeness in all national industrial relations systems, the variations are immense. This article probes into the nature of representativeness, displaying its significant effect on the ability of workers to negotiate, on the capacity of trade unions to achieve collective agreements, and on the distributive effects of such agreements. The article concludes that the constitutional dimension of representativeness must be considered, demonstrating rare instances of robust constitutional challenges and directing attention to the seemingly more mundane forms of response, legal or extra-legal, that actually shape the course of their incremental legal development.