We report the fabrication of three-dimensional structures of submicrometer resolution by three-photon polymerization. This resolution has been achieved by polymerizing ORMOCER, a UV photocurable organic-inorganic hybrid material, with an ultrafast laser irradiation at 1028 nm. To our knowledge, this is the first demonstration of three-photon polymerization, a process that may allow the fabrication of components of very high resolution.