Nonmuscle cells exert biomechanical forces known as traction forces on the extracellular matrix (ECM). Spatial coordination of these traction forces against the ECM is in part responsible for directing cell migration, for remodeling the surrounding tissue scaffold, and for the folds and rearrangements seen during morphogenesis. The traction forces are applied through a number of discrete adhesions between a cell and the ECM. We have developed a device consisting of an array of flexible, microfabricated posts capable of measuring these forces under an adherent cell. Functionalizing the top of each post with ECM protein allows cells to attach and spread across the tops of the posts. Deflection of the tips of the posts is proportional to cell-generated traction forces during cell migration or contraction. In this chapter, we describe the microfabrication, preparation, and experimental use of such microfabricated post array detector system (mPADs).