Abstract: X‐ray crystallography studies on fragments D and double‐D from human fibrinogen and fibrin have revealed the details of knob‐hole interactions between fibrin units, as well as the nature of the association at their ends. More recently, a lower‐resolution structure of native chicken fibrinogen has provided details about the structure of the central domain, and particularly the arrangement of disulfide bonds. Parts of the fibrinogen molecule are so flexible that they have not been visualized in electron density maps. The elusive regions include the αC domain, the amino‐terminal segments of the α and β chains, and the carboxyl‐terminal segments of the γ chains. Nonetheless, when all the structural data are considered together, it is possible to construct a realistic model not only of a fibrinogen molecule but also of a fibrin protofibril.