This paper explores the implications of the increasing role of informal as well as formal markets in the health systems of many low and middle-income countries. It focuses on institutional arrangements for making the benefits of expert medical knowledge widely available in the face of the information asymmetries that characterise health care. It argues that social arrangements can be understood as a social contract between actors, underpinned by shared behavioural norms, and embedded in a broader political economy. This contract is expressed through a variety of actors and institutions, not just through the formal personnel and arrangements of a health sector. Such an understanding implies that new institutional arrangements, such as the spread of reputation-based trust mechanisms can emerge or be adapted from other parts of the society and economy. The paper examines three relational aspects of health systems: the encounter between patient and provider; mechanisms for generating trust in goods and services in the context of highly marketised systems; and the establishment of socially legitimated regulatory regimes. This analysis is used to review experiences of health system innovation and change from a number of low income and transition countries.