When light travels through a diffusing medium, the incident radiation is scattered and a part of it is reemitted. The reflected proportion is defined as the reflectance and provides the quantification of color sensations. Many authors have tried to derive reflectance values by various means. This bibliographical synthesis reviews the main theories on the subject, comparing them and especially their assumptions and derivations. First, the widely used theory of Kubelka and Munk?which has been proved to be a particular case of Schuster's formula?is described, according to its terms, limits, improvements, and applications. Next, the well-known electromagnetic approach by Mie is presented, along with methods derived from radiative transfer theory, multilayer methods (the "pile of plates'' due to Stokes), multiflux methods (developed by Mudgett and Richards or Völz for instance), and the corpuscular treatment proposed by Silvy. Finally, new methods are listed that allow the calculation of reflectance values, such as Monte Carlo simulations, expert systems, or neural networks. A chronological organization chart is also given to place each work or advance relative to the others.