The facility of leeches for extracting blood from living organisms has long been known to human society. That influential ancient physicians such as Nicander of Colophon (ca. 200 B.C.), Wang Chung (ca. A.D. 30), and Kunja Lal Sharma (ca. A.D. 50) described therapeutic uses for leeches indicates that medical use of leeches was widespread in the ancient world. However, application of leeches in Europe for relief of a myriad of ailments such as headaches, insomnia, ulcers, and obesity did not reach its acme until the 1800s, when exploitation in France exceeded 50 million leeches per year. Today, legitimate use of medicinal leeches includes hematoma treatment after microsurgical reattachment procedures. There has also been an increase in understanding of the anticoagulation components of leech salivary secretions. Thorough understanding of the evolutionary relationships of leeches provides a means for targeting other therapeutically useful species and permits the elucidation of patterns in the evolution of blood feeding from their common ancestor to the present.