The aqueous boundary layer (ABL) affects various aspects of oral absorption of a drug, from dissolution of the drug to the apparent K(m) value of intestinal wall metabolism and carrier mediated transport. However, the importance of ABL has often been entirely ignored in oral absorption investigation. In this minireview, the effect of ABL on oral absorption of a drug is discussed in an easy-to-understand manner. This review starts with an introduction of the boundary layer theory with many illustrations (and links to public web movies visualizing the ABL), and then discusses some specific cases of interest in pharmaceutical science, such as dissolution of floating drug particles in the USP paddle apparatus. The effect of the boundary layer on the membrane permeation is also discussed from the viewpoint of structure permeability relationship, carrier mediated transport/metabolism and estimation of the fraction of a dose absorbed for poor solubility compounds.