Fibrinogen is a complex multifunctional protein comprised of three major domains (two outer D and one central E) which contains constitutive binding sites (e.g. Da, Db, γXL, D:D, γ', thrombin substrate, platelet receptor) as well as binding sites that become exposed or expressed as a result of fibrinogen proteolysis by thrombin and/or that are exposed as a consequence of the polymerization process itself (tPA binding sites). Fibrin-dependent tPA-mediated activation of plasminogen is associated with exposure of polymerization-dependent epitopes (Aα148–160, γ312–324) that are expressed in assembled fibrin and in crosslinked (polymerized) fibrinogen but not in unpolymerized fibrinogen or fibrin. Fibrin polymerization is initiated by thrombin cleavage of fibrinopeptide A from fibrinogen Aα chains, exposing two E domain EA sites. Cleavage of fibrinopeptide B from fibrinogen Bβ chains exposes other E domain polymerization sites, termed EB, that also interact with platelets, fibroblasts and endothelial cells. Fibrin generation is followed by an assembly process of intermolecular end-to-middle D to E associations to form linear and branched double-stranded fibrin fibrils, lateral fibril-fibril associations to form fibers and a branched fiber network. Binding sites in fibrinogen play their roles in fibrin assembly by self-association (γXL to γXL and D:D to D:D) or by complementary association with exposed sites in fibrin (Da to EA and Db to EB). Other binding sites in fibrinogen include thrombin substrate recognition sites in each E domain and a non-substrate high affinity thrombin binding site in the carboxy-terminal region of each γ' chain, which also binds plasma factor XIII. Fibrin possesses low affinity thrombin binding sites in each E domain and retains the γ' chain non-substrate thrombin-binding site.