Coumarin is a structural fragment of different natural and synthetic compounds which demonstrate a wide range of pharmacological activities. Its derivatives are of interest in many fields because of their properties as oral anticoagulants or rodenticides, photosensitizers, anti-HIV agents and antibiotics. There has been continuous interest in the synthesis of these compounds. This book describes various experiments for modifying the structure of clinically used anticoagulants, and establishes the structure of the synthesized products through X-ray analysis. It also identifies unexpected structures in this regard, and highlights the usefulness of all these structural modifications through different biological, toxicological and pharmacological experiments.