The growth of new blood vessels, or angiogenesis, is a naturally occurring process in both health and disease states. An area of active research, regulation of angiogenesis, is being studied as an approach for the treatment of cancer and a range of other disorders having vascular proliferation as a component. The process of angiogenesis is very complex and occurs in multiple steps, with a major involvement of endothelial cells. Various in vivo models have been developed to assess inhibitors of angiogenesis. As these are generally technically difficult and labor intensive, with observed effects difficult to quantify, they do not lend themselves to compound screening. Rather they are used for confirmatory studies. In contrast, in vitro assays developed to model various steps in the angiogenesis process are easy to perform and lend themselves to high-throughput analysis. Described in this unit is an in vitro assay that can be employed to investigate endothelial differentiation inhibitors through assessment of their effects on capillary tube formation by endothelial cells on Matrigel.