Insects counteract infection by a variety of reactions, partly humoral but principally cellular. This monograph considers their cellular reactions, especially the phagocytosis of micro-organisms and the encapsulation of larger parasites, from two main points of view: parasitological and cytologica. The first aspect involves description of the reactions and of their effects on parasites. This part of the subject is basic to the biological control of insect pests, because a better understanding of cellular defence reactions could lead to improved methods of using insect parasites to human advantage. The second aspect involves analysis of the stimuli that evoke cellular reactions. This part of the monograph attempts to relate the defensive activities of insect blood cells to general problems of cytology, such as the recognition of foreign bodies, the aggregation of cells and their adhesion to foreign surfaces and their extreme flattening on each other as they form capsules. Two final chapters discuss the efficiency and specificity of insect defence mechanisms and compare them with the immunity reactions of vertebrates.