Horse chestnut seed comes from a large, ornamental, deciduous tree that is cultivated around the world, particularly in Europe and Russia. The tree is familiar to many children in these countries as the source of “conkers”—a nickname for the large, shiny brown seeds. Traditionally, horse chestnut seed has been used to treat conditions such as neuralgia (nerve pain), rheumatism, varicosities (swollen and inflamed veins), and other conditions of venous congestion accompanied by dull pains, fullness, and engorgement (swelling). In addition, it has been used for rectal complaints such as haemorrhoids, rectal neuralgia, and proctitis (inflammation of the anus and rectum). However, there is relatively little modern evidence for the use of unprocessed plant products. In Germany there has been much professional use of and research on proprietary horse chestnut seed extracts, and in 1996 these extracts comprised the third-best selling category of herbal medicines in that country. These extracts are standardised to a saponin fraction referred to as aescin (or escin). Clinical trials support the efficacy of the extracts in treating chronic venous insufficiency (chronic swelling and inflammation of the veins), oedema (swelling) and haematoma (bruising), as well as for preventing post-surgical thrombosis (blood clots). In addition to various standardized products for oral use, topical preparations for bruises and swollen tissues are available for external use. IMPORTANT NOTE: In this monograph, all ratings and most other information relate to proprietary products, as the evidence supporting their efficacy largely eclipses that derived from traditional use. The traditional use summary may be consulted for guidance on the use of unprocessed preparations, but note that the safety of these preparations has not been as well monitored and horse chestnut is sometimes included, perhaps only theoretically, in lists of poisonous plants.