The phenomena of scattering and absorption of light and other electromagnetic radiation by small particles and particle groups are central to a great variety of science and engineering fields. Owing to a large body of research, the discipline of studying these phenomena has recently undergone profound and paradigm-shifting developments. Among the most important advances are the following:

• Dramatic improvements in numerical solvers of the Maxwell equations coupled with the ever-growing computer capability have enabled direct, numerically exact modeling of electromagnetic scattering by particles and particle groups of unprecedented morphological complexity.

• The rigorous physical basis of monochromatic and polychromatic scattering by random particles and random particle groups has been established.

• Owing to the development of a rigorous microphysical approach, the centuries-old disciplines of directional photometry and radiative transfer have become legitimate branches of physical optics.

• Direct computer solutions of the Maxwell equations have confirmed the mesoscopic origin of radiative transfer and weak localization of electromagnetic waves (also known as coherent backscattering) in sparse particulate media.

The main purpose of this textbook is to provide a self-contained and accessible summary of these developments in the framework of a thorough introduction to the fundamental physical and mathematical principles of the subject. Particular attention is paid to key (and often overlooked) aspects, such as time and ensemble averaging at different scales, ergodicity of stochastic scattering objects, and the physical nature of measurements afforded by actual directional photometers and photopolarimeters.