This chapter lays the land for physically modeling light-matter interactions in rendering, discussing the different levels at which the modeling can occur. We will then introduce one such model, an extremely high-level model that abstracts away almost all the underlying physics and models an object as its apparent reflection and transmission spectra. This simple modeling is functionally very useful, as it is very commonly used in practice, and serves as a curious teaser for the remaining chapters: how and when can this simple model be a good approximation of the sophisticated light-matter interactions? We will conclude with a brief overview of radiometry, which provides the necessary analytical tools we will use in physical modeling.