CD40 is a cell surface receptor, which belongs to the TNF-R family, and which was first identified and functionally characterized on B lymphocytes. However, in recent years it has become clear that CD40 is expressed much broader, including expression on monocytes, dendritic cells, endothelial cells and epithelial cells. Therefore it is now thought that CD40 plays a more general role in immune regulation. The present paper 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; 4) in vivo functions of CD40/CD40-L interactions.