After a short introduction to the classification of medicinal leeches, their historical use in phlebotomy (blood-letting) and contemporary use in neurobiology and medicine are summarised. Over-collecting of wild Hirudo medicinalis in Europe led to reduced populations and the need to import other species, especially the closely related Hirudo verbana from Turkey and, more recently, the Caribbean and Asian leech, Hirudinaria manillensis. The limited information on the quantitative ecology of European medicinal leeches is summarised next. They require warm-water ponds with a range of suitable hosts, especially amphibians, to survive and prosper. Medicinal leeches can persist with a low minimum viable population size, which may be typical of rare freshwater invertebrates in isolated habitats, especially species limited by high temperature requirements and specialised food sources. Phylogenetic relationships, using molecular methodology, show that there are at least two independent lineages of medicinal leeches with Hirudo medicinalis, H. verbana and Hirudinaria manillensis being closely related. The type species, H. medicinalis, was once abundant in Europe but is now rare and on the endangered list in several countries. Genetic studies have confirmed the erroneous marketing of H. verbana as H. medicinalis. It is highly probable that H. verbana has already escaped into the wild. Unlike H. medicinalis, H. verbana has no legal protection. We conclude that the major factor in the decline of medicinal leech populations has been the general loss of wetlands, especially eutrophic ponds and marshes throughout Europe. Destruction of these water bodies has also led to a decline in amphibians that are an important source of blood-meals for the leeches and are crucial for the survival of their juveniles. More quantitative information is required on H. medicinalis, and especially H. verbana, to facilitate their conservation and management, and to prevent them becoming extinct in the wild.