This chapter starts from the classical optics description of a light field and its spectral densities, their measurement, and their interpretation, and discusses how these aspects are related to the quantum properties of a single light mode. It reviews ways to measure these quantum properties. Emphasis is placed on Gaussian states of a light mode, i.e. states for which the Wigner function has a two-dimensional Gaussian shape. The discussion continues with a description of more than one mode, and a unifying approach to quadratic Hamiltonians is presented, including phase conjugation, which is related to time reversal.