Although scientists have discovered many fundamental physiological and behavioral mechanisms that comprise the stress response, most of current knowledge is based on laboratory experiments using domesticated or captive animals. Scientists are only beginning, however, to understand how stress impacts wild animals - by studying the nature of the stressful stimuli that animals in their natural environments have adapted to for survival, and what the mechanisms that allow that survival might be. This book summarizes, for the first time, several decades of work on understanding stress in natural contexts. The aim is two-fold. The first goal of this work is to place modern stress research into an evolutionary context. The stress response clearly did not evolve to cause disease, so that studying how animals use the stress response to survive in the wild should provide insight into why mechanisms evolved the way that they did. The second goal is to provide predictions on how wild animals might cope with the Anthropocene, the current period of Earth’s history characterized by the massive human remodeling of habitats on a global scale. Conservation of species will rely upon how wild animals use their stress response to successfully cope with human-created stressors.