The development of efficient mass-rearing systems for biological control agents (BCAs) is crucial for the biological control (BC) of exotic pests in forest plantations. BCAs can be reared in the laboratory to facilitate establishment and ensure a wider distribution for routine release in the field. Optimization of mass rearing procedures is thus also important in this context. Rearing conditions need to be economical and produce large numbers of high-quality individuals. Rearing systems should simulate the insect microhabitat and its overall environment. In this chapter, advancements and progress within the context of the rearing systems of parasitoids used in BC of forest pests are documented. Attention is especially directed to the basic needs of rearing systems, including the facilities required and the factors affecting the host insects and parasitoids. A brief overview of the rearing systems for the most important parasitoids used in BC programmes in forest plantations, and insights on the challenges and recommendations for research, are also provided.