1.1. Introduction to Nonlinear Optics1.2. Descriptions of Nonlinear Optical Processes1.3. Formal Definition of the Nonlinear Susceptibility1.4. Nonlinear Susceptibility of a Classical Anharmonic Oscillator1.5. Properties of the Nonlinear Susceptibility1.6. Time-Domain Description of Optical Nonlinearities1.7. Kramers-Kronig Relations in Linear and Nonlinear OpticsProblemsReferences