When an intense light interacts with a material, the material responds nonlinearly to the field amplitude. In this section, basic concepts of nonlinear optical processes are presented in Sect. 5.1. Then several nonlinear processes important in high power lasers are described: frequency conversion (Sect. 5.2), optical parametric amplification including optical parametric chirped pulse amplification (Sect. 5.3), self-focusing and self-phase modulation (Sect. 5.4), and propagation of extremely short, intense laser pulses (Sect. 5.5).