This is a brief review of the major events which take place when plasmin reacts with fibrinogen (fibrinogenolysis) and fibrin (fibrinolysis). An effort is made to interrelate the locations of the major products of fibrinogenolysis (X, Y, D and E) in the originating fibrinogen molecule. The complexity of the lysates of fibrins, cross-linked by factor XIII to varying extents, is discussed. Clinical and structural implications of two fragments (D dimer and the D dimer-E complex) unique to cross-linked fibrin digests, are highlighted.