Tissue factor is now widely accepted to be the major physiological initiator of blood coagulation. The importance of this protein to normal haemostasis may be inferred from the clinical effects of factor VII deficiency and by the fact that congenital tissue factor deficiency has not been described. Tissue factor is a single polypeptide with an established amino acid sequence that may be divided into cytoplasmic, extracellular and transmembrane regions. There are no close structural similarities with other proteins although tissue factor may be a member of the cytokine receptor family. This paper reviews current knowledge of the role of tissue factor in the production of thrombin with particular reference to its expression, location, control and relevance to haemostasis.