Coupled waves are used in many fields of modern optics: volume holograms, Bragg reflectors, acousto-optic modulators, waveguide couplers, distributed feedback lasers, polarization effects in optical fibers and liquid crystals, non-linear optics, parametric amplifiers and oscillators, four-wave mixing, etc. First, the general concept of coupled waves is introduced for any kind of perturbation (spatial and temporal) of the dielectric polarization within an optical medium. The result is a general form of coupled wave solutions. Second, the particular topics mentioned above are treated by introducing the special conditions for the basic modes to be considered (plane waves, guided waves, optical polarizations) and the physical effects relating the induced perturbation of the dielectric polarization to the electrical field (spatial and temporal variation of refractive index, absorption or birefringence, non-linear optical effects, etc.). This approach puts the emphasis on teaching concepts rather than presenting particular effects. The fundamental role of phase- matching (Bragg) and the similarities of the solutions for different physical effects emerge clearly.