It can be argued that predation is the fundamental process of ecology. It follows that ecologists must understand how the predation rate depends on the populations involved, of both predator and prey, to understand the behavior of food webs. The theme of this book is that this dependence has been largely misunderstood, with consequences for ecological theory and its applications. In particular, in most current predator–prey and food web models, the per capita predation rate, or predator functional response, is assumed to be a function of prey density alone, and is thus termed prey-dependent (PD).