The conversion of the plasma protein fibrinogen to the biopolymer fibrin is catalyzed by the serine protease thrombin. The resulting structural properties of this biopolymer and the effect of the “clotting environment” on the fibrin gel morphology (fiber thickness, gel porosity, branch points) will be discussed. Emphasis will be given to fibrin gels formed with fibrin sealant preparations. In addition, the mechanical properties of these gels will be discussed, particularly the effect of FXIII and fibrinogen concentration.