The class deals with the interaction of light with matter. Classical phenomenology and Maxwell`s equations in media are used to describe the nonlinear interaction and propagation. Frequency conversion and intensity dependent refractive index as well as scattering processes are described. Nonlinear processes important for modern lasers and diagnostics like optical Kerr effect and correlation methods are explained. Finally, the present day high field nonlinear optics will be highlighted on the action of free electrons in a strong laser field.