Thrombin is the primary promoter of blood clotting; it also plays an important role in the regulation of the coagulation cascade and has been implicated in a number of other cellular processes. How can one molecule catalyse such a variety of events? The recent X-ray structure determination of human a-thrombin and related structures shows that the molecule can be divided into several functional regions that recognize different chemical moieties. By using different combinations of these elements, thrombin can interact with a variety of macromolecules with high specificity.