This book provides a synthesis of the physical, physiological, evolutionary, and biomechanical principles that underlie animal locomotion. An understanding and full appreciation of animal locomotion requires the integration of these principles. Toward this end, we provide the necessary introductory foundation that will allow a more in-depth understanding of the physical biology and physiology of animal movement. In so doing, we hope that this book will illuminate the fundamentals and breadth of these systems, while inspiring our readers to look more deeply into the scientific literature and investigate new features of animal movement. Several themes run through this book. The first is that by comparing the modes and mechanisms by which animals have evolved the capacity for movement, we can understand the common principles that underlie each mode of locomotion. A second is that size matters. One of the most amazing aspects of biology is the enormous spatial and temporal scale over which organisms and biological processes operate. Within each mode of locomotion, animals have evolved designs and mechanisms that effectively contend with the physical properties and forces imposed on them by their environment. Understanding the constraints of scale that underlie locomotor mechanisms is essential to appreciating how these mechanisms have evolved and how they operate. A third theme is the importance of taking an integrative and comparative evolutionary approach in the study of biology. Organisms share much in common. Much of their molecular and cellular machinery is the same. They also must navigate similar physical properties of their environment. Consequently, an integrative approach to organismal function that spans multiple levels of biological organization provides a strong understanding of animal locomotion. By comparing across species, common principles of design emerge. Such comparisons also highlight how certain organisms may differ and point to strategies that have evolved for movement in diverse environments. Finally, because convergence upon common designs and the generation of new designs result from historical processes governed by natural selection, it is also important that we ask how and why these systems have evolved.