Fibrinogen contributes to thrombosis risk in different ways. Indeed, various mutations in the fibrinogen genes predispose to thrombosis. At the same time, high levels of fibrinogen are also associated with thrombotic complications. Although the underlying causative mechanisms are not clear, this most likely involves the associated inflammatory and hypercoagulable states. In the last few years, particular attention has focused on the polymorphisms of fibrinogen genes involved in increased fibrinogen levels or fibrinogen qualitative changes. The association between dysfibrinogenemia and risk of thrombosis is well known, and some mechanisms have been clearly identified. Paradoxically, some patients with either hypofibrinogenemia or afibrinogenemia may also suffer from severe thromboembolic complications. The management of these patients is particularly challenging because they are not only at risk of thrombosis but also of bleeding. This review discusses the various quantitative and qualitative defects of fibrinogen associated with thrombosis, the tests that may predict the thrombotic risk, as well as some preventive or therapeutic approaches.