The vessel wall is composed of heterogeneous cell populations residing in a variety of vascular beds. Each cell type has different functions and morphologies but all of them have a role in the repair process following vascular injury. Responses to injury vary depending upon the type and extent of the injury and the vascular bed affected. The sheet migration and proliferation exhibited by large vessel endothelial cells is in striking contrast to the migration through soft tissues and tube formation exhibited by microvascular endothelial cells in response to injury. Vascular smooth muscle cells respond to injury by migrating into the intima, proliferating and synthesizing matrix, causing intimal thickening. The response to injury by vascular cells appears to be modulated, in part, by the composition and organization of the surrounding matrix and the various platelet factors and cytokines found at sites of injury. Furthermore, evidence has been accrued in culture, suggesting that solid phase (matrix) and soluble factors modulate each other's effects on local vascular cell populations following injury.