This chapter contains sections titled: Introduction Homocysteine Metabolism Homocysteine Forms In Vivo Homocysteine Measurement Causes of Hyperhomocysteinemia Therapeutic Options for Lowering Elevated Homocysteine Epidemiologic Evidence Linking Homocysteine and Atherothrombotic Vascular Disease Homocysteine and Atherothrombosis: Pathophysiologic Mechanisms Impact of Homocysteine-Lowering Therapy on Atherothrombotic Vascular Disease Conclusions References