Recent progress in three-dimensional microstructuring of photosensitive materials by femtosecond lasers has been reviewed. Due to the nonlinear nature of multiphoton laser-activated processing, application of ultrashort laser systems allows one to overcome the diffraction limit and to produce high-quality 3D microstructures with a subwavelength resolution. This is very powerful technology with many potential applications which have been briefly discussed in this chapter. This 3D nonlinear laser processing technique is still in a rapidly developing phase and represents a very exciting field of laser physics and novel laser material processing technologies.