This book is motivated by the idea that the cost of inaction can be much greater than the cost of action. Inaction can lead to serious negative consequences—for individuals, the economy, and society. The consequences of a failure to reduce extreme poverty, for example, typically include malnutrition, preventable morbidity, premature mortality, incomplete basic education, and other human and social development costs. In this volume, the authors seek to clarify exactly what is meant by “cost of inaction.” They develop a methodology to account for the consequences and estimate the costs of a failure to respond to the needs of children and their families. Their conceptual framework emphasizes the need to select appropriate actions against which inaction is evaluated. The authors present the results of applying the cost of inaction (COI) approach to six case studies from Rwanda and Angola. The case studies highlight important differences between the COI approach and benefit-cost analysis as it is traditionally implemented.