It has been more than 20 years since it was first demonstrated that endothelial cells will rapidly form capillary-like structures in vitro when plated on top of a reconstituted basement membrane extracellular matrix (BME, Matrigel, EHS matrix, etc.). Subsequently, this morphological differentiation has been demonstrated with a variety of endothelial cells; with endothelial progenitor cells; and with transformed/immortalized endothelial cells. The differentiation process involves several steps in blood vessel formation, including cell adhesion, migration, alignment, protease secretion, and tubule formation. Because the formation of vessel structures is rapid and quantifiable, endothelial cell differentiation on basement membrane has found numerous applications in assays. Such differentiation has been used (1) to study angiogenic and antiangiogenic factors, (2) to define mechanisms and pathways involved in angiogenesis, and (3) to define endothelial cell populations. Further, the endothelial cell differentiation assay has been successfully used to study processes ranging from wound repair and reproduction to development and tumor growth. The assay is easy to perform and is the most widely used in vitro angiogenesis assay.