Modern drug design not only focuses on the pharmacological activity of a co
mpound but also considers its ability to be absorbed and to reach its site
of action. The prediction of in vivo barrier permeabilities, particularly i
ntestinal absorption and blood-brain barrier passage, are substantial conce
rns in the development of new drug compounds. For several decades, n-octano
l-water partition coefficients dominated absorption prediction, In recent y
ears, the basic physicochemical parameters describing both membrane permeab
ility and lipophilicity have been established. This review provides an outl
ine of some selected absorption-prediction models with emphasis on intestin
al absorption.