This book is about animal locomotion as viewed from the perspectives of comparative animal biophysics, biomechanics, and bioengineering. Animal biophysics may be defined as the study of biological phenomena, structures, and processes in animals, including intact animals, by using the concepts and methods of physics. The field overlaps broadly with and complements comparative animal physiology, much of which emphasizes chemical and biochemical approaches to animal function. Animal biomechanics and bioengineering may be defined as the application of mechanical and other engineering principles to the study of the same animals, phenomena, structures, and processes (Leondes 2007–2009; Chien et al. 2008; Chien 2013).