We present an approach to fabricate an array of elastomer posts in order to dynamically measure the traction forces exerted by living cells on a surface with a micrometer lateral resolution. Arrays of closely spaced vertical microposts are made in silicone elastomer [poly(dimethylsiloxane) (PDMS)] by molding a Silicon substrate that has been machined by deep Si etching after standard photolithography. The surface of the micropillars was modified to allow cell culture. Deflections of the calibrated posts were dynamically followed by direct obervation with an optical microscope. By using this set-up, we could dynamically draw up a cartography of the local traction forces exerted by the cells.