This volume presents ecological consequences and evolutionary mechanisms that may be associated with behavioral alterations in parasitized hosts. Alterations may result from natural selection favoring the host or parasite, or there may be side effects of physiological changes associated with symbiosis. This book summarizes the literature in this area, showing that reports of alteration may frequently be consistent with more than one evolutionary explanation and that rigorous tests are needed before the phenomenon can be understood and placed in a predictive framework. Such understanding is important, because these behavioral alterations have been shown to have the potential to affect the distribution of animals in nature and their interactions with predators and competitors, and the alterations may have implications for vector biology.