In this review we have summarized the large mass of information that has accumulated in recent years relative to the heterogeneous group of molecules known as the interleukins (IL), the lymphokines, and the cytokines that control the growth and differentiation of cells of the hematopoietic and lymphoid lineage and cells of other lineages. Our intent is to provide the informed generalist with a body of information with which to interpret and understand forthcoming studies of this important class of molecules, particularly those with clinical import.