The text develops the basic principles that underlie nonlinear optical phenomena in matter, in an approach oriented to the general reader or student with a background in classical electromagnetic theory. It begins with a discussion of linear wave propagation in dispersive media, moves into weak nonlinearities which can be discussed in a pertuberative manner, then examines strong nonlinear effects (solitons, chaos). The emphasis is on the macroscopic description on nonlinear phenomena, within a semi-classical framework. The text is aimed at the student or researcher who is not a specialist in optics, and who wishes to acquire an introduction to the principal concepts as a precursor to the consultation of the specialized literature.