Identifying the mechanisms regulating angiogenesis in pathologic conditions such as cancer and heart disease is crucial to develop successful therapies. The dependence of angiogenesis on characteristic properties of these conditions, such as alterations in tissue stiffness due to changes in the composition of the extracellular matrix (ECM), may shed light on potential therapeutic strategies. Prior studies have suggested that ECM compliance regulates capillary morphogenesis, but the mechanisms remain unclear. In this study, we hypothesized that ECM density, which influences substrate mechanics, may regulate angiogenesis via a mechanism involving actin-mediated cell-generated forces. To investigate this hypothesis, we utilized an in vitro model of angiogenesis in which endothelial cells coated on microcarrier beads are distributed within a 3-D fibrin ECM. A monolayer of fibroblasts, which provides pro-angiogenic factors, is cultured on top of the gel. Variations in fibrin gel density along with a library of pharmacologic agents that inhibit forces generated by the actin cytoskeleton were used to prove the necessity of cell-generated tractional forces in blood vessel formation. Our data demonstrate that cell-generated forces not only play a crucial role in the early sprouting stages of capillary morphogenesis, but are also required in the later maintenance stages of capillary morphogenesis, and thereby suggest a broader interdependence between tissue stiffness, cell contractile forces, and angiogenesis.