This chapter studies subsurface scattering and volume scattering. While superficially different, they involve the same forms of light-matter interaction and are modeled in the same way. We start with an example to build some useful intuitions, and then we will get into the weeds of modeling. In our modeling, we start from modeling local events (absorbing and scattering photons by particles), from which we will build a general framework, called the Radiative Transfer Equation and its variant Volume Rendering Equation (VRE), to reason about subsurface and volume scattering globally. Finally, we will connect VRE to (neural) radiance-field rendering, a modern iteration of image-based rendering (Sect.  5.3.7 ) that uses VRE to parameterize the image formation process.