After defining oral absorption, this review article summarizes the influence of different physiological and physicochemical parameters on oral absorption explained according to the situation at BayerScheringPharma. Along the optimization process in industries, prediction of oral absorption is exemplified. In silico prediction of oral absorption in early stages of research is highlighted with three examples, i.e., a classification algorithm, a single parameter prediction, and a linear free-energy relationship. In vitro prediction of permeation using Caco-2 cell layers and physicochemical modeling is exemplified. The influence of solubility is shown by a concrete example, in particular, using physiologically based pharmacokinetic (PBPK) modeling and physicochemical parameters. For later stages, optimization examples of in vivo animal models stress the importance of excellent in vivo data and in vivo/in vivo correlations for the prediction of human oral absorption from animal experiments.