Thrombophilia is a term recently coined to describe the increased tendency of some patients to develop thrombosis. Although the term was originally used as a clinical description, the abnormalities of the coagulation system responsible can now be identified in approximately 50% of cases of thrombosis. Many of these disorders are hereditary and it is this group that are generally referred to as thrombophilia. Recent studies have begun to establish the mutations responsible for the hereditary thrombophilias and to assess their interactions with one another. However, it is also clear that many other factors remain undiscovered.