Fibrinogen is a soluble plasma protein that is converted to polymeric fibrin in response to damage to the vascular system. The clotting process is initiated when platelets aggregate at the wound site. Their disruption releases biologically active amines and a proteolytic cascade follows which culminates in the conversion of fibrinogen to fibrin. The fibrin polymer forms the matrix of the tangle of cellular and molecular substances called the blood clot. Atomic-level details are now in hand for many of the interactions that hold fibrin units together, although some aspects have yet to be resolved. Of necessity, fibrin clots need to be dismantled when they are no longer needed, an operation largely accomplished by the proteolytic enzyme plasmin. Various regulatory phenomena are involved in maintaining the balance between intravascular fluidity and clots that prevent blood loss. A variety of hereditary conditions, including mutant fibrinogens, can predispose individuals to either thrombosis or bleeding. Key concepts: The underlying fabric of blood clots is a protein polymer called fibrin. Fibrin clots are formed in response to injuries to any part of the vascular system. The conversion of soluble fibrinogen molecules to insoluble fibrin depends on thrombin generated from prothrombin. Keywords: fibrinogen; fibrin; clot stabilisation; fibrinolysis; blood clotting; X-ray structures