A monograph is devoted to an important but rarely seen in literature problem a parasitism as derivative of biosphere evolution. The following questions are studied: a transformation of parasitism features during an organic world evolution; a gnoseology of parasitism; definitions and essence of concepts and terms; concepts of parasitism; parasites' environments: its unity and non-equivalence; a role of pathogeny of parasitic organisms in evolution of an organic world; basic forms of ecological relationships (mutualism, procooperation, commensalism, etc.); a variety of forms of relationships between organisms in natural surroundings; an introduction into scientific vocabulary and semantic dualism of 'parasitary system' concept: its epidemiological and biological understanding; a structural and functional basis of parasitary systems organization; effects of climate change on parasitary systems; concept of paleoparasitology. The authors view a parasitology as having manifold capabilities as an heuristic instrument for study of more fundamental questions of biology and see too low demand on parasitological data in this respect. The book is meant for biologists of various specialties.