Parasitism is a very common way of life, and probably the prevalent means of obtaining food among organisms. Adaptive radiation among parasites has been extensive, and yet ecological and evolutionary concepts on parasitism are poorly developed. Although Elton (1927) devoted a chapter in his pioneering ecology book to parasitism his conclusion was that the resemblances between predators and parasites are more important than the differences. This attitude is now prevalent in ecology texts (e.g. Andrewartha and Birch, 1954; Odum, 1971; Krebs, 1972; Colinvaux, 1973), and although much attention is devoted to predation, parasitism is almost ignored. Therefore, by using the inductive process, this paper attempts a synthesis of ecology and parasitology, the need for which has been recognized by Kennedy (1975), and it explores the evolutionary implications of parasite ecology. This synthesis should contribute to an understanding of parasites in three ways. First, a reevaluation of the abundance of parasitic species is made. Arndt (1940) estimated that 25% of animals in Germany are parasitic on others. Rothschild and Clay (1952) stated that parasitic animals probably exceed nonparasitic species in number of species and individuals, but provided little numerical support. A careful quantitative evaluation by Askew (1971) provided an estimate that 15% of insects, and 10% of all animal species are parasitic insects. Calculations presented in this paper indicate that well over 50% of the species of organisms extant today are parasitic. Second, general concepts on the ecology and evolution of parasites are presented, being derived from a combination of population biology, evolutionary theory and parasite biology. Third, predictions derived from the general concepts act as a guide to critical characteristics of parasite biology which need to be examined and tested in natural populations.