The synthesis of peptides as imitations of the thrombin cleavage site of fibrinogen has led to sequences with affinity for the enzyme. These peptides were first developed as chromogenic and fluorogenic substrates for thrombin. The same idea was also used to generate peptide substrates for other serine proteases in blood coagulation and fibrinolysis. Amidolytic methods based upon the substrates have revolutionized assays of proenzymes, enzymes, cofactors and inhibitors in research as well as in clinical laboratories. Like peptidomimetics based only on Arg (P1 of the natural substrate) or Arg analogues, these amino acid sequences also have been developed as active site directed inhibitors of thrombin and of factor Xa. A further interesting development is the synthesis of bivalent thrombin inhibitors which, like hirudin, bind to the thrombin active centre as well as to its anionic exosite. Recently, also, it has been shown that the positively charged side chain of P1 Arg is not an absolute necessity for binding a peptide to the active site of thrombin. Several of the new thrombin inhibitors show interesting properties for pharmaceutical development and some of them are on clinical trials.