This chapter provides an overview of the factors that govern the absorption of chemical substances in humans. Emphasis is on those physicochemical properties of the substance and the anatomical and physiological characteristics of the skin, gastrointestinal tract, and lung that are most significant in relation to absorption. No discussion of the other factors (i.e., extent and duration of exposure, metabolism, distribution, excretion, etc.) important to bioavailability of chemical substances appears here (for additional discussion of these factors, see Benet et al., 1996; Bronaugh, 1990; Hrudey et al., 1996; Medinsky and Klaassen, 1996; Rozman and Klaassen, 1996; Wright, 1995). Unless stated otherwise, the terms “chemical substance” or “substance” refer to non-electrolyte organic substances. The goal of this chapter is to provide an approach to accurately estimate absorption of substances for which little or no absorption data exist.