This chapter begins with a recap of the free space Maxwell’s equations, the density of the electromagnetic energy, and the Poynting vector for the energy flux. In order to separate the Maxwell equations, a vector potentialVector potential is introduced in addition to the usual scalar potential. FluctuationsFluctuation in the electric permittivity are identified as the cause of the scattering of the incident light. It is then shown that both the vector potential and the scalar potential can be combined in terms of a Hertz potentialHertz potential, which can be used to calculate the electric and magnetic fields that determine the intensity of the scattered light. The Hertz potential is shown to satisfy an inhomogeneous wave equationWave equation, which is solved by the Green’s[aut]Green, George functionGreen’s function method. The Rayleigh ratio is introduced in order to quantify the wavelength and angular dependence of the scattering. The turbidityTurbidity follows as the angle average of the Rayleigh ratio. The theory of fluctuations, that was introduced in Chap. 2 , is then invoked in order to identify the specific material properties associated with the scattering. Specific systems analyzed for their light scattering powers include the ideal gas, a real fluid with a liquid–vapor critical point, a binary liquid mixture with a critical point of solution, and finally a solution where the solute is a macromoleculeMacromolecule.