The urokinase plasminogen activator receptor (u-PAR), present on the surface of a number of cell types, is a key component in the plasminogen activation system where it serves to localize and accelerate the activation cascade. In the present review, structural and functional aspects of u-PAR are discussed with special emphasis on newer findings. The structural features discussed include a repetitive domain structure with an NH2-terminal, ligand-binding domain, a membrane anchor of the GPI-type and high amounts of N-linked carbohydrate. The demonstration of a previously identified monocyte activation antigen, Mo3, as being identical with u-PAR, is described. Functional aspects include the ligand-binding process, the consequences of this binding process for the activation cascade system, the cleavage of u-PAR by u-PA itself, and the internalization of u-PA inhibitor complexes. Finally, a number of findings are discussed which support the view that u-PA/u-PAR are important components in degradation processes during cancer invasion. Immunocytochemistry and in situ hybridization studies have revealed complicated interplays between different cell types, specifically at the invasive front regions of tumours, with regard to these molecules, and in in vitro model systems for invasion the putative roles of the components have been studied.