The corpus luteum (CL) is an organ that exhibits extremely rapid growth, development, and regression during the course of each nonpregnant cycle. The CL consists of steroidogenic (parenchymal) and nonsteroidogenic (nonparenchymal) cells. The small and large parenchymal cells differ in numerous morphological and functional characteristics, and are thought to interact with each other to maintain normal luteal function. These steroidogenic luteal cells also interact with the nonsteroidogenic cells; for example, they produce factors that stimulate proliferation and migration of endothelial cells and proliferation of fibroblasts; they also may enhance or suppress immune cell function. Conversely, endothelial cells produce factors that modulate steroidogenesis, and immune cells produce cytokines that modify the secretory function of steroidogenic cells. Cellular interactions may be mediated by several mechanisms, including humoral (endocrine and paracrine) pathways as well as contact-dependent (gap junctional) pathways. Thus, hormones, growth factors and cytokines produced locally by steroidogenic or nonsteroidogenic cells may be transferred from cell to cell indirectly or directly to regulate luteal function. Gap junctions are present in luteal tissues of several species, and gap junctional intercellular communication is affected by the stage of luteal development and systemic and local regulators of luteal function. Such cellular interactions probably are important in luteal hormone production, signal transduction, angiogenesis, and luteolysis because of their role in coordinating function among the various luteal cell types.