The collagens belong to the constituents of the subendothelium that determine the thrombogenicity of the vessel wall. The existence of several genetically distinct collagens is well documented. To date, 19 collagens have been characterized. The collagens are divided into the fibril forming collagens and the non-fibril forming collagens. At least nine of the different collagens--type I, III, IV, V, VI, VIII, XII, XIII, XIV--were found in the vessel wall. All collagen molecules are built of three chains in a triple-helical conformation. Some collagens also contain large parts of non-collagenous domains. An interesting example for complex collagens is collagen type VI for it shares non-collagenous domains with the von Willebrand factor A domain, the platelet glycoprotein Ib, fibronectin type III repeats, and Kunitz type protease inhibitor. Monomeric and fibrillar collagens effectively support platelet adhesion, whereas for collagen-induced platelet aggregation and secretion, the native, triple-helical structure of collagen is required. The platelet reactive sites in collagens type I and III have been studied intensively. Using CNBr-peptides three aggregatory sites in the alpha 1(I) chain and one in the alpha 1(III) chain have been found. Cyanogen bromide fragments of collagen type I were also used to measure platelet adhesion under flow conditions. alpha 1(I)CB3 strongly supports platelet adhesion. The two peptides alpha 1(I)CB3 and alpha 1(III)CB4 are highly homologous. alpha 1(III)CB4 is highly aggregatory. Several substances are known to interfere at different levels with the platelet-collagen interactions and have been identified in blood-feeding animals. Among them are the "leech anti-platelet protein" (LAPP), calin, moubatin, and pallidipin.