The articles in this Special Issue of Social Science & Medicine document the infl uence of the growing involvement of civil society in the health sector internationally. They examine the multiple, and, in some cases contested, roles that civil society organisations (CSOs) can play in response to both demand-related strains on health systems and changes in the role and structures of the state. To set a context for the articles which follow, this introduction situates civil society and CSOs generally within the contemporary crisis of health systems and considers their role as a tool of health governance. It also draws attention to the contested theoretical and defi nitional issues surrounding research and practice on the role of CSOs in the delivery of care and health advocacy and their place in health systems reform.