To address the question of how cell-generated forces regulate the organization and function of endothelial cells, I investigated the mechanical forces in a simple model of cell-cell contact: paired cells contacting each other via a single cell-cell junction. To study the responsiveness of AJs to force, I adapted a system of microfabricated force sensors to quantitatively report both the cell-cell tugging force and the size of adherens junctions (AJ). I observed that AJ size was modulated by tugging force: AJs and tugging force grew or decayed with myosin activation or inhibition, respectively. This myosin-dependent regulation operated in concert with a Rac1, force-independent control of AJ size, and was illustrated by showing that effects of vascular permeability agents (S1P, thrombin) on junctional stability are reversed by changing the extent to which these agents coupled to the Rac and myosin-dependent pathways. Furthermore, I showed that direct application of mechanical tugging force, rather than myosin activity per se, is sufficient to trigger AJ growth. Inspired by the study of mechanical force on the junctions between two cells, I further extended our tools to measure mechanical force in a more complicated multicellular system involving more cells and more than one type of cells. I investigated the role of mechanical force in a model system of monocytes transmigrating across an endothelial monolayer. Using our force measurement system, I first found that the average traction force in the whole endothelial monolayer increased during monocyte firm adhesion and transmigration. By specifically look at traction forces at the cellular level, I found that the endothelial cell with the monocyte firmly adhered on it showed a much larger traction forces, with the direction of the traction force aligned more centripetally toward the monocyte. Moreover, the sub-cellular and pan-cellular analysis of the traction forces in the monolayer revealed an increase of traction force in local zones vicinity to the monocyte. Finally, engagement of endothelial adhesion molecules could increase traction forces in the endothelial cells. Taken together, this study implicates mechanical forces in firm adhesion and transmigration.