Cellular forces are closely related to many physiological processes, including cell migration, growth, division, and differentiation. Here, we describe newly developed techniques to measure these forces with high spatial resolution. Our approach is based on ultrasoft silicone elastomer films with a regular microstructure molded into the surface. Mechanical forces applied by living cells to such films result in elastomer deformation which can be quantified by video microscopy and digital image processing. From this deformation field forces can be calculated. Here we give detailed accounts of the following issues: (1) the preparation of silicon wafers as molds for the microstructures, (2) the fabrication of microstructured elastomer substrates, (3) the in-depth characterization of the mechanical properties of these elastomers, (4) the image processing algorithms for the extraction of cellular deformation fields, and (5) the generalized first moment tensor as a robust mathematical tool to characterize whole cell activity. We present prototype experiments on living myocytes as well as on cardiac fibroblasts and discuss the characteristics and performance of our force measurement technique.