It is of great importance to predict in vivo pharmacokinetics in humans based on in vitro data. We summarize recent findings of the quantitative prediction of the hepatic metabolic clearance from in vitro studies using human liver microsomes, hepatocytes, or P450 isozyme recombinant systems. Furthermore, we propose a method to predict pharmacokinetic alterations caused by drug-drug interactions that is based on in vitro metabolic inhibition studies using human liver microsomes or human enzyme expression systems. Although we attempt to avoid the false negative prediction, the inhibitory effect was underestimated in some cases, indicating the possible contribution of the active transport into hepatocytes and/or interactions at the processes other than the hepatic metabolism, such as the metabolism and transport processes during gastrointestinal absorption.