CD40 is a cell surface receptor that belongs to the tumor necrosis factor‐R (TNF‐R) family, and that was first identified and functionally characterized on B lymphocytes. Its critical role in T cell‐dependent humoral immune responses was demonstrated by patients with the hyper‐IgM syndrome, as well as by gene targeting in mice. However, in recent years it has become clear that CD40 is expressed much more broadly, including expression on monocytes, dendritic cells, endothelial cells, and epithelial cells. In addition, the CD40‐ligand (CD40‐L/CD154), a member of the TNF family, is also expressed more widely than activated CD4⁺ T cells only. Therefore it is now thought that CD40‐CD40‐L interactions play a more general role in immune regulation. Collectively these studies have culminated in pre‐clinical and clinical studies that are in progress. This article reviews recent developments in this field of research, with main emphasis on (1) structure and expression of CD40 and its ligand; (2) CD40 signal transduction; (3) in vitro function of CD40 on different cell types; and (4) in vivo functions of CD40/CD40‐L interactions. J. Leukoc. Biol. 67: 2–17; 2000.