Dysfibrinogens can be grossly divided in two groups: (1) defective thrombin-catalyzed conversion of fibrinogen molecules to fibrin monomers, and (2) defective fibrin polymerization due to structural alterations in polymerization sites, that include "A" and "a" sites, end-to-end D:D abutment surfaces, and lateral association sites involving the carboxyl terminal region of the fibrin alpha-chain. Recently, a number of mutations in the fibrinogen genes have been identified, and many of these encode changes that occur in regions of fibrinogen that have been elucidated by high-resolution structural studies. Here we focus on the structure-function relationships of fibrinogen that can be inferred from studies involving these abnormal molecules.