The discovery of factor XII (Hageman factor), the attempts of characterizing it, the knowledge of its presence of absence in the various vertebrate classes are represented in a survey and recent opinions of molecular biology about its structure and composition are discussed. The effect of the activating substances for transferring the Hageman factor into its active form are represented with the role of the pre-kallikrein (Fletcher factor) and of the highly molecular kininogen (Fitzgerald factor) being referred to. Furthermore, the following systems influenced by factor XII are dealt with: 1. The endogenous and exogenous activating system of blood clotting and possible reasons for lower bleedings with factor XII deficiency. 2. Role of the Hageman factor in activating fibrinolysis. 3. Influence on the liberation of kinin. 4. Correlations towards the complement system. 5. Possible inhibitory effect of platelet aggregation. Finally the close connection of all these systems is referred to and the necessity of considering these complicated events in a complex way is stressed.