Quantitative measurements of cell-generated forces have heretofore required that cells be cultured on two-dimensional substrates. We describe a technique to quantitatively measure three-dimensional traction forces exerted by cells fully encapsulated within well-defined elastic hydrogel matrices. We apply this approach to measure tractions from a variety of cell types and contexts, and reveal patterns of force generation attributable to morphologically distinct regions of cells as they extend into the surrounding matrix.