The history of the word "leech" and the practice of leeching reveal interconnected social histories. We give the linguistic and medical histories of the word, and explore its biology and clinical history. Our historical account extends from the earliest known record of leeching to current research. Despite historical variation in its reputation as a therapeutic technique, leeching remains useful today in a number of applications. Further investigation may well disclose even more uses for the leech, particularly for its enzymes with anesthetic, anticoagulant, and antimetastatic properties.