This book assesses the value and relevance of the literature on complex systems to policy-making, contributing to both social theory and policy analysis. For this purpose it develops two key ideas: agile action and transformative realism. The book takes some major themes from complexity science, presents them in a clear and accessible manner and applies them to core problems in sociological theory and policy analysis. Combining complexity science with perspectives from institutionalism and political economy, this book is the first to integrate these fields conceptually, methodologically and in terms of the implications for policy analysis and practice. Room shows how the models and methods of social and complexity science can be jointly deployed and applied to empirical areas of public policy. He demonstrates how complexity science can provide insight into the nonlinear dynamics of the social world, but why these need to be understood by reference to the unequal distribution of power and advantage. Among the sociological debates with which the book engages are those concerned with causation and explanation, rational action and positional competition, and the place of evolutionary concepts in accounts of social change. Among the policy debates are those concerned with evidence and policy, the dynamics of inequality, and libertarian paternalism. The book will appeal to final year undergraduates and postgraduate students in social sciences; scholars in social and policy studies broadly defined; policy-makers who want to go beyond conventional discussions of evidence-based policy-making and cross-national lesson-drawing, and consider how to approach complex and turbulent policy terrains; and a wider range of scholars in other disciplines where complexity science is already well developed.