The problems of the biochemical and physiological aspects of the host-parasite relationship have been explicitly separated from consideration of the physiology and biochemistry of the parasite. Certainly, the metabolic activities of the parasite will in part determine the nature of the relationship but it is most important that we recognize the host-parasite as a physiological system. This idea was, as far as I can determine, first enunciated by De Bary in 1879 when he defined symbiosis, and may have been recognized before that time. Host-parasite relationships, as scientific problems, may be resolved as two types of research: those which are reductionist in character and attempt to define the molecular relationships between two organisms of different species, and those which are integrative and attempt to define host-parasite relations at the supraorganismal level in quantitative terms. In the present context it is appropriate to begin at the level of the reductionist and examine some molecular relationships between host and parasite. A few typical relationships have been chosen for discussion in some detail.