Parasitic or parasitoid Hymenoptera are widely used for biological control; indeed, over 360 species representing 18 families have been used in control programs. 1 Although many details of their biology obviously differ, the vast majority of hymenopteran parasitoids share one feature in common — their genetic system of haplodiploidy — in which they also differ from most other insects. In this chapter, I discuss various features of parasitoid genetics which arise as a consequence of haplodiploidy and are relevant (although not exclusively so) to biological control. The topics to be covered are (1) genetic variation; (2) sex determination and diploid males; (3) sex ratios and mating systems; (4) geographical variation, host-parasite coevolution, and sexual dimorphism; and (5) extrachromosomal inheritance.