Nonlinear spectroscopy results when a light field perturbs the optical properties of a molecule so that the subsequent light fields see changes in the molecular states. It is important that the different light fields interact with the molecular states over time periods that are shorter than periods associated with dephasing and population relaxation so that the effects of the first interaction are not lost. By exciting a state with one light field and probing it at a later time, nonlinear experiments allow one to investigate how quantum states evolve in time and how different states are related to each other. We must first understand the evolution of coherences and populations that characterize the different nonlinear methods. We use second order spectroscopies as an example.