Some groups function well, from the perspectives of equity, efficiency, and well-being, while others do not. This book explores why. It covers groups that perform three types of function: overcoming market failures (e.g. producer organizations); improving the position of their members (e.g. Trade Unions), and distributing resources to the less well-off (e.g. NGOs and the public sector). 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.