This book is divided into three sections. The first is concerned with the ecological nature of some of the problems faced by animals as a particular consequence of their adopting a parasitic mode of life, and the strategy, rather than the tactics, of their solutions to these problems. The second section is concerned with the specific problems and difficulties posed by the major organs and tissues occupied by parasites, and the tactical solutions adopted by parasites in each of these habitats that enable them to cope with and overcome these difficulties. The third section is concerned with population ecology, and emphasises that although individual parasites have to face and overcome particular problems, parasitism is essentially a dynamic relationship between two species populations.