In this review we will examine the characteristics of the various cell types which have been shown to present antigens to helper and proliferating T cells and explore what is known about the requirements for antigen presentation by these cells. Cell types to be discussed include mononuclear phagocytes from a variety of tissues as well as nonphagocytic cells such as Langerhans cells and dendritic cells. Special consideration will be given to the most recent group of cells to have demonstrated antigen-presenting capacity, B lymphocytes. Experiments exploring the processing and presentation of antigen by these different cell types will be presented. These results suggest that immunologically relevant antigen is endocytosed and at least partially degraded before proper presentation can occur. The role of molecules synthesized by the presenting cells, such as MHC antigens and cytokines, will be discussed in detail.