Whereas the adhesion and migration of individual cells have been well described in terms ofphysical forces, the mechanics of multicellular assemblies is still poorly understood. Here,we study the behavior of epithelial cells cultured on microfabricated substratesdesigned to measure cell-to-substrate interactions. These substrates are covered by adense array of flexible micropillars whose deflection enables us to measure tractionforces. They are obtained by lithography and soft replica molding. The pillardeflection is measured by video microscopy and images are analyzed with home-mademultiple particle tracking software. First, we have characterized the temporaland spatial distributions of traction forces of cellular assemblies of various sizes.The mechanical force balance within epithelial cell sheets shows that the forcesexerted by neighboring cells strongly depend on their relative position in themonolayer: the largest deformations are always localized at the edge of the islands ofcells in the active areas of cell protrusions. The average traction stress rapidlydecreases from its maximum value at the edge but remains much larger than theinherent noise due to the force resolution of our pillar tracking software, indicatingan important mechanical activity inside epithelial cell islands. Moreover, thesetraction forces vary linearly with the rigidity of the substrate over about twodecades, suggesting that cells exert a given amount of deformation rather thana force. Finally, we engineer micropatterned substrates supporting pillars withanisotropic stiffness. On such substrates cellular growth is aligned with respect to thestiffest direction in correlation with the magnitude of the applied traction forces.