Serpins have a central role in regulating proteolysis in blood coagulation, fibrinolysis, and inflammation. Congenital serpin deficiencies cause specific clinical syndromes such as thrombosis with antithrombin III deficiency or emphysema with alpha 1-antitrypsin deficiency. Acquired serpin abnormalities have also been reported, for example, increased PAI-1, which is believed to represent a major risk factor for the development of thromboembolic complications. This article has reviewed the structure/function relationships of serpins and have presented ideas concerning their mechanism of action. The implications of this information for designing drugs that could interfere with serpin function has also been described, and mention has been made of the therapeutic potential of such compounds.