Publisher Summary Hemocyte numbers and subpopulations change in response to both wounding and the introduction of foreign material into the hemocoel. The mechanism whereby hemocytes recognize wounded self may be the basis upon which more refined recognition mechanisms have been added. Hemocytes show a gradation of response towards wounded self and towards non-self, indicating that the system can discriminate degrees of difference from self; the extent of the hemocytic response is clearly under careful control. This chapter describes several types of cellular immune response and the ways in which they may relate to the “humoral” response. Three particular aspects of hemocytic behavior are also examined: (1) changes in populations, subpopulations and in individual hemocytes as a result of wounding and the introduction of foreign material into the hemocoel; (2) the initiation of these changes either directly by hemocytic recognition of non-self or wounded self or indirectly by the interaction of hemocytes with soluble factors deriving from the area of non-self or wounded self; and (3) the controlled augmentation of the resultant hemocytic response. Because the degree of recognition and response is also affected by the insect's prior experience, several factors that suppress or enhance the response are also discussed in the chapter.