This book sets out a framework for adaptive explanations of behavior, and uses it to provide analyses of a range of biological issues such as energetic gain, energy-predation trade-offs, dynamic games, state-dependent life histories, annual routines, and fluctuating environments. The volume gives a definitive account of this exciting new field, summarizing previous research, presenting new material, and suggesting directions for future research. The framework adopted in this book provides a common currency for comparing diverse actions. Ultimately this could lead to the development of state-dependent dynamic models. This book will be a revelation for graduate students and researchers interested in behavioral or evolutionary biology.