In this review, I set forth some basic information about the mechanical properties of fibrin clots and attempt to identify the big questions remaining. The intent is to make this topic understandable to both basic scientists who are interested in blood clotting and to hematologists or cardiologists, since I believe that this is something everyone working in these fields should know. The viscoelastic properties of fibrin are remarkable and unique among polymers. Moreover, these properties are essential to the physiology of blood clotting and are important for understanding and therefore preventing and treating thrombosis.