The spectra of Brillouin scattering processes in optical fibers are affected by temperature, axial strain, and other quantities of interest. This dependence forms the basis for optical Brillouin scattering based optical fiber sensors. Since the first proposition of such sensors in 1989, several protocols have been established for the spatially distributed analysis of Brillouin scattering spectra along fibers installed in structures of interest. Sensor systems cover hundreds of kilometers, reach sub-millimeter resolution, follow dynamic vibrations at MHz rates, and resolve sub-degree temperature changes and micro-strain elongations. Optical fiber sensors represent the most successful commercial application of Brillouin scattering physics to-date. This chapter reviews the principles, state of the art, performance trade-offs and recent breakthroughs in Brillouin scattering-based optical fiber sensors.