This book focuses on group behaviour in developing countries. It includes studies of producer and community organizations, NGOs, and some public sector groups. Despite the fact that most economic decisions are taken by people acting within groups, the analysis of group functioning has not received enough attention, particularly among economists. Some groups function well, from the perspectives of equity, efficiency, and well-being, while others do not. It covers groups that perform three types of function - overcoming market failures; improving the position of their members, and distributing resources to the less well-off. It contrasts three modes of group behaviour - power and control; cooperation; and the use of material incentives. It explores what determines modes of behaviour of groups, and the consequences for efficiency, equity, and well-being.