Publisher Summary This chapter discusses immune system in insects. In insects, both cellular and humoral immune responses figure prominently in host defense pathways, with many parasitic and pathogenic infections resulting in the deployment of both cellular and humoral defenses, which act synergistically. Insect hemocytes participate in phagocytosis, nodulation, and encapsulation processes. Humoral defenses include production of antibacterial, antifungal, and antiviral molecules, the melanizing enzyme phenoloxidase and its toxic reactive precursors, agglutinins, serpins, immulectins, and many other humoral mediators of immune defense. Insect immunology is a rapidly maturing discipline, with exciting prospects for novel applications in control of vector-borne diseases and agricultural pests, biotechnology, and medicine. The outcome of the interactions between a parasite or pathogen and its host is determined by a combination of the susceptible vs. refractory status of the host and the virulant/avirulant characteristics of the infecting agent. By manipulation with biotechnology, insect immune responses to parasitoids, pathogens, or symbionts, the vectors of plant and animal diseases can be disarmed, thereby curbing disease transmission. Similarly, the economic impact of agricultural pests can be reduced by enhancing pest susceptibility to pathogenic bacteria, viruses, and fungi, as well as development of biotechnological approaches to manipulating the host range of beneficial parasitoids.