This chapter discusses absorption spectroscopy as it might relate to near-infrared (NIR) spectroscopy. This is a rigorous treatment of the two-flux approach to modeling diffuse scatter as it applies to absorption spectroscopy. It covers the topics that NIR spectroscopists have been exposed to as theory over the last few decades but is really just several mathematical models. It begins with the absorption spectroscopy of solutions and covers several two-flux models of scattering samples, as can be applied to uniform particulate samples. In Chapter 2, the authors provided an overview of physical principles that underlie the observed absorption and scattering that occur in spectroscopy. This chapter focuses on the construction of mathematical models. Unfortunately, there is no single, uniformly applicable mathematical treatment that “works” for all applications (whether one is thinking of spectroscopy in general or near-infrared (NIR) spectroscopy in particular). Consequently, the intent of this chapter is to place a particular emphasis on the assumptions made in the development of each model. The usefulness of a model depends upon how well the assumptions made in formulating the model align with the physical realities of the case at hand.