Acquired fibrinogen deficiency is a major determinant of severe bleeding in different clinical conditions, including cardiac surgery, trauma, postpartum hemorrhage, liver surgery, and transplantation. The existing guidelines recommend to supplement fibrinogen in patients with severe bleeding when the fibrinogen concentration is <1.5 g/L. Viscoelastic tests (VETs) provide a fast determination of the fibrinogen contribution to clot firmness and allow prompt treatment of acquired fibrinogen deficiency. However, different VET devices are presently available on the market, based on different technologies and different activators and platelet inhibitors. The available tests are the functional fibrinogen (FF, thromboelastography), the fibrinogen contribution to clot firmness (fibrinogen determination [FIBTEM], thromboelastometry), and the fibrinogen contribution to clot strength (FCS, sonorheometry). All these tests have a moderate to very good correlation with the Clauss fibrinogen assays; however, when comparing VET-based fibrinogen contribution to clot firmness with Clauss fibrinogen concentration, strong differences occur within the same test under different conditions and between different tests. The most widely studied test is the thromboelastometric FIBTEM; the best predictor of a Clauss fibrinogen <1.5 g/L is placed at a maximum clot firmness around 8 mm of amplitude. Fewer data are available for thromboelastographic FF, but the correspondent value is in the range of 12 mm. Overall, due to an incomplete inhibition of platelet contribution, FF overestimates the fibrinogen contribution with respect to FIBTEM. Data on sonorheometry FCS are limited and conflicting. When addressing the correlation between different tests, it is good in general, but no fixed conversion factors can be proposed, due to a considerable dispersion of the experimental points. In conclusion, VET-based fibrinogen tests are certainly powerful tools that are presently suggested by the existing guidelines; however, when using them for clinical decision-making, users should consider the possible sources of bias, which include the different level of platelet inhibition, the role of platelet count and function, the possible different degrees of blood activation with tissue factor, the important role of factor XIII in stabilizing the fibrin clot, and others.