Advances in molecular biology describing important enzyme systems involved in drug conjugation and deconjugation reactions and recent work indicating the importance of drug and xenobiotic conjugates as transport forms of biologically active compounds are reviewed comprehensively. Part One describes molecular events associated with the expression and regulation of transferases and hydrolases involved in Phase II drug conjugation and deconjugation. Part Two deals with the regulation of Phase II conjugation, and Part Three reviews critically the importance of drug conjugates in pharmacology and toxicology. This volume is an up-to-date source of information on this topic and will be of broad interest to pharmacologists and toxicologists.