The chemotherapy of malaria is complicated by the fact that no anti-malarial drug so far discovered is therapeutically effective against all stages of human malaria. Not only do malarial parasites differ in their reactions to drugs at different stages in their complicated life cycles, but different species of parasites at comparable stages of development show different responses, and finally, strains within a given species may show significant differences in susceptibility to treatment. Furthermore, the treatment of patients in whom immunity may be disregarded is not necessarily that most suitable for the partially immune and indigenous patients of a malarious region. These matters must be kept in mind if treatment is to be intelligently applied, and as a basis some understanding of modern views regarding the development of malaria in man is necessary. Malarial Cycle in Man There are four human species of malarial parasite, namely, Plasmodium fakiparum, which produces the severe and dangerous malignant tertian malaria, P. vivax, the parasite of benign tertian malaria, P. malariae, producing quartan malaria, and P. ovale, with which is associated tertian fever resembling that of vivax malaria. These parasites follow the same general pattern of development, but P. falciparum shows certain differences which have an important bearing on treatment.