Hereditary fibrinogen disorders (HFDs) are a heterogeneous group of rare inherited abnormalities of fibrinogen. HFDs are caused by monoallelic or biallelic mutations in FGA, FGB, and FGG genes on chromosome 4. The spectrum of molecular abnormalities is large, leading to several subtypes of fibrinogen disorders with particular biological and clinical characteristics. HFDs are classified according to functional and antigenic fibrinogen levels: quantitative disorders include afibrinogenemia (complete absence of fibrinogen) and hypofibrinogenemia (proportional decrease of functional and antigenic fibrinogen levels); qualitative disorders include dysfibrinogenemia (decreased functional and normal antigenic fibrinogen levels) and hypodysfibrinogenemia (discrepant decrease of functional and antigenic fibrinogen levels). Besides fibrinogen levels, HFDs are classified by the clinical phenotype and the genotype into several subtypes. An accurate diagnosis is fundamental to providing tailored treatment according to the clinical setting.