Leeches possess properties that make them uniquely able to assist with venous compromised tissue. Their saliva contains an anticoagulant and a histamine-like vasodilator that promote local bleeding, a local anesthetic, and hyaluronidase that promotes the local spread of the other leech salivary secretions into the wound/bite. In addition, active pharyngeal peristalsis further promotes the egress of venous blood. Resurgence in the use of leeches has been stimulated by Upton in the United States and Mahaffey in Europe. Currently, leeches are used at many microsurgical centers to provide critical venous outflow for compromised tissue replantations and transfers that might otherwise be unsalvageable. As the use of leeches becomes more widespread, knowledge of leech biology and physiology is important. This review reports on Hirudo medicinalis, the species used most often medically in Europe and the United States.