Light scattering by particles is the process by which small particles cause optical phenomena, such as rainbows, the blue color of the sky, and halos. Mie scattering defines the interaction of light with particulate matter with a dimension comparable to the wavelength of the incident radiation. It can be regarded as the radiation resulting from a large number of coherently excited elementary emitters (molecules for example) in a particle. Since the linear dimension of the particle is comparable to the wavelength of the radiation, interference effects occur. The most noticeable difference to Rayleigh scattering is, generally, the much weaker wavelength dependence and a strong dominance of the forward direction in the scattered light. The calculation of the Mie scattering cross section, which involves summing over slowly converging series, is complicated even for spherical particles, it is worse for particles of an arbitrary shape. However, the Mie theory for spherical particles is well developed and a number of numerical models exist to calculate scattering phase functions and extinction coefficients for given aerosol types and particle size distributions.