Nonlinear properties include intensity-dependent refractive index, multiphoton absorption, second and third harmonic generation, stimulated Raman and Brillouin scattering, and associated phenomena such as self-focusing, self-phase modulation, optical phase conjugation, and optical bistability. We report on a large amount of work that reaches beyond simple glasses such as silica. Nonlinear optical phenomena have been investigated in materials ranging from oxide, halide, and chalcogenide glasses to glasses doped with semiconductor microcrystals, organic dyes, and metal particles and in forms varying from bulk materials to optical fibres and waveguides. We also discuss recently discovered photoinduced effects in glass fibres, which include second harmonic generation and the formation of refractive index gratings