Book excerpt: Many drugs and other xenobiotics (e.g., preservatives, insecticides, and plastifiers) contain hydrolyzable moieties such as ester or amide groups. In biological media, such foreign compounds are, therefore, important substrates for hydrolytic reactions catalyzed by hydrolases or proceeding non-enzymatically. Despite their significance, until now, no book has been dedicated to hydrolysis and hydrolases in the metabolism of drugs and other xenobiotics. This work fills a gap in the literature and reviews metabolic reactions of hydrolysis and hydarion from the point of views of enzymes, substrates, and reactions.