Substantial evidence exists which implicates the urokinase plasminogen activator system [urokinase plasminogen activator (uPA), urokinase plasminogen activator receptor (uPAR) and plasminogen activator inhibitor-1 (PAI-1)] in the neo-vascularization, invasion and metastasis of many solid tumors. Clinical studies have demonstrated an association between high levels of expression of the components of this system in tumors and poor patient prognosis and outcome. Components of the uPA/uPAR system are differentially expressed or activated on motile cells including invading tumor cells and leukocytes, and migrating endothelial cells. In contrast, there is little or no expression on most normal, quiescent cells. Studies performed in vitro have demonstrated the regulation of the expression of uPA and uPAR by growth and differentiation factors as well as by oncogenes. In this review, we summarize recent findings on the role of the components of the uPA/uPAR system in angiogenesis, invasiveness and tumor metastasis. The activities of this system in endothelial and leukocyte cell biology and the relevance of these activities to angiogenesis and tumor metastasis will be considered. Recent experimental evidence obtained using inhibitors of uPA and uPAR has validated this system as a therapeutic target for the development of anti-angiogenic and anti-metastatic therapeutic agents. These studies, as well as additional therapeutic and diagnostic implications for uPAR targeting, will be discussed.