Publisher Summary One would like to use the lowest possible level of abstraction in the design of biophysically based local lighting models so that even small perturbations in the biophysical processes scan be accounted for. In practice, the appropriate abstraction level is usually tied to data availability. First-principles models attempt to work on the lowest level of abstraction for which there is available data. For instance, instead of using a volumetric absorption coefficient for a given tissue, their formulation can incorporate specific absorption coefficients for the tissue constituents. Many models used in computer graphics rely on spectral parameters. There are measured reflectance curves for human skin available in the biomedical literature, but they are limited to a number of skin types and restricted to a narrow range of illuminating and viewing angles. Furthermore, measured transmittance curves for the skin organ as a whole are scarce. These aspects highlight the need to develop models of light interaction with human skin that can compute not only its scattering properties (usually given in terms of BRDF and BTDF), but also its spectral properties. BioSpec was the first model to use a first-principles approach for computing both spectral and scattering quantities for skin specimens. For this reason, this chapter closely examines its design and evaluation.