Leeches are blood sucking worms that have been used in medicine as far back as 2500 years ago. The word leech comes from an old English word ‘laece’ meaning ‘physician’. The spelling later became leech.  In medieval England, leeches were linked with healing because of the etymology of the word. In old English the word “lacian” meant to heal and physicians were known as “leche”. Leeches are annelids or segmented worms. All leeches have 34 body segments. In medieval and early modern medicine, the medicinal leech- Hirudo medicinalis and its congeners was used to remove blood from a patient as part of a process to “balance” the “humours”. Hirudotherapy was introduced by Ibne Sina in the Canon of Medicine (1020s). He considered the application of leech to be more useful than cupping in “letting of the blood from deeper parts of the body”. The secretion of leech saliva contains like Hirudine, Histamine, Hyluronidase, Collgenase, Fibrinases, Hementin, Bdellin, Eglins, Elastase, Cathepsin, Inhibitor of Kellikerin, Anesthetics, Protinase inhibitor, Tryptase inhibitor, Antibacterial. Key words: Leeches, Hirudotherapy, humours, Hirudine, Histamine, Hyluronidase