The propagation of intense optical beams through dielectric media induces changes in the refractive index which cause self-focusing and beam breakup in high-power laser systems. After a brief discussion of the origin and spatial anisotropy of refractive-index nonlinearities, methods for measuring the nonlinear refractive index coefficient are reviewed. The use of time-resolved interferometry is described in detail. Nonlinear indices for optical glasses and crystals measured at 1064 nm by this technique are tabulated. From these data, an empirical expression is given which, using the linear refractive index and partial dispersion, provides a good estimate of the nonlinear index of optical materials in the long-wavelength limit.