Computational oral absorption simulation (COAS) is anticipated to be a powerful tool in improving the productivity of drug discovery and development. This article reviews the theories of pharmaceutical sciences that consist of COAS. Although most of these theories are classical, they are revisited from the context of modern drug discovery and development. The theories of solubility, diffusion, dissolution, precipitation, intestinal membrane permeation and gastrointestinal transit are comprehensively described. Prediction strategy is then discussed based on the biopharmaceutical classification system. In the final part, good simulation practice is proposed and many frequently asked questions answered.