Allee effects are broadly defined as a decline in individual fitness at low population size or density, that can result in critical population thresholds below which populations crash to extinction. As such, they are very relevant to many conservation programmes, where scientists and managers are often working with populations that have been reduced to low densities or small numbers. There are a variety of mechanisms that can create Allee effects, including mating systems, predation, environmental modification, and social interactions among others. The abrupt and unpredicted collapses of many exploited populations is just one illustration of the need to bring Allee effects to the forefront of conservation and management strategies. This book provides an overview of the topic, collating and integrating a widely dispersed literature from various fields: marine and terrestrial, plant and animal, theoretical and empirical, academic and applied.