Unani system is a rich source of medicinal herbs and uses number of regimental therapies for curing the diseases. Irsal-e-Alaq (Leech therapy), one of the most important regimental therapy; a widely practiced method, is known from the time of extreme antiquity and is still alive today. This fact testifies its efficiency in various health problems. Eminent Unani physicians Razi, Majoosi, Zahrawi, Ibne Sina, have described and practiced the use of nonpoisonous (or medicinal leeches) in a number of diseases including joint diseases, chronic non-healing ulcers and various skin disorder like eczema, psoriasis etc. Recent, scientific studies also reveal that leech saliva contains hirudin and histamine etc. which have shown anticoagulant effect; hence improve microcirculation in diseases like varicose veins and diminished edema. Therefore, leech therapy can be used as an alternative or adjacent therapy for these diseases. And need of the hour is to generate clinical data for validation of claims of Unani physicians about efficacy of this therapy. Present paper deals with the historical aspect from Unani literature and brief scientific activities of leech therapy.