In all OECD countries, workers and employers associate to express their interests and concerns, as well as to bargain over the terms and conditions of employment. However, over the last decades, the share of workers who are trade union members and the reach of collective agreements even beyond union membership have significantly declined. Despite these trends, collective bargaining, together with workers’ voice, continues to play a key role in the labour market in many OECD countries. Collective bargaining systems are generally based on a complex set of rules and practices, partly written in national laws and partly based on longstanding traditions. In order to understand their functioning and role, it is necessary to look at their different building blocks, and their interactions. These include: The actual level of bargaining (i.e. the level at which bargaining takes place, firm vs. sectoral level, and the margins firms have to derogate or to opt out from higher-level agreements); the degree of co-ordination across sectors and bargaining units; the effective enforcement of collective agreements, and the overall quality of labour relations.