Metabolic research and especially that which focuses on the monooxygenase cytochrome P450 system has gone through dramatic progressive changes within the last decade. Availability of highly sophisticated analytical techniques has contributed to increased molecular understanding of biotransformation pathways. Nowadays designer drugs are computer composable focussing on metabolic stability. The rapid development of recombinant enzymes has, in many cases, replaced the intensive work on P450 isolation from naturally occurring sources. Recombinant P450s can be expressed in cell lines and thus made available for reproducible and constant use addressig a large number of scientific questions. This book describes the present position of such research.