The theory of reflectance and emittance spectroscopy is based on the fundamental principles of radiative transfer (the propagation of energy in the form of electromagnetic radiation) in particulate media. This chapter outlines key models for radiative transfer in particulate media that can be forward-modeled to predict reflectance and emittance spectra or inverted to obtain the abundance of geologic materials from remote observations. The models are rooted in the optical properties of geologic materials, namely the complex index of refraction, and the scattering of light controlled by particulate texture, shape, and size. The chapter is divided into reflectance modeling and emittance modeling because of the key difference in the origin of the electromagnetic radiation: external to the grain’s surface and internal to the grain’s surface, though the principles are common across this division. The key models presented for reflectance spectroscopy are the Hapke model for scattering and reflectance and the Shkuratov model for reflectance. For emittance spectroscopy, the Hapke model forms the backbone upon which hybrid models incorporating Mie T-matrix concepts are integrated.