The main idea behind this book is to present a rigorous derivation of the equations that govern light propagation in highly scattering media, with an emphasis on their applications in imaging in biology and medicine. The equations and formulas for diffuse light propagation are derived from the very beginning, and all the necessary analytical expressions needed to complete a complex imaging or characterization problem are presented step by step. This book provides postgraduate and PhD students with the basic framework and sufficient knowledge in light transport and the related mathematical methods to solve any complex problems that may appear in any biomedical applications involving multiple scattered light. All results presented are formal analytical derivations from the complete problem, presenting, in those cases which are relevant, approximations to these expressions. In this sense, numerical solutions to these expressions, such as the finite element methods, are not within the scope of this book.