For almost a half century scientists have striven to develop a theoretical model capable of predicting oral drug absorption in humans. From the pH-partition hypothesis to the compartmental absorption and transit (CAT) model, various qualitative/quantitative approaches have been proposed, revised and extended. In this review, these models are classified into three categories; quasi-equilibrium models, steady-state models and dynamic models. The quasi-equilibrium models include the pH-partition hypothesis and the absorption potential concept, the steady-state models include the film model and the mass balance approaches, and the dynamic models include the dispersion, mixing tank and CAT models. The quasi-equilibrium models generally provide a basic guideline for understanding drug absorption trends. The steady-state models can be used to estimate the fraction of dose absorbed. The dynamic models predict both the fraction of dose absorbed and the rate of drug absorption and can be related to pharmacokinetic models to evaluate plasma concentration profiles.