none. Cyzenis albicans; Agrypon flaveolatum; Operophtera brumata. This chapter is divided into four parts. First, the basic model structure is outlined, indicating why this is well suited to parasitoids and especially to some biological control programs. Second, the question is then posed: "What accounts for the persistence (= stability) of host and parasitoid populations at very low average levels of abundance (= equilibrium levels) following a successful program?" Third, a further question is then asked: "Does theory support the practice of multiple parasitoid introductions?" Finally, a case study is briefly examined, showing how a simple model can contribute to an understanding of an observed biological control outcome.