Acquired coagulation factor antibodies are either alloantibodies or autoantibodies. Alloantibodies are formed when the body reacts to an external antigen. Commonly, alloantibodies arise when a patient who has a congenital clotting factor deficiency is infused with a blood product. Alternately, patients exposed to a coagulation protein from a different species may develop alloantibodies to the animal protein that cross-reacts with their own protein. On the other hand, autoantibodies develop spontaneously in people without pre-existing factor deficiencies and without exposure to an external antigen. These antibodies may neutralize function of a clotting factor, promote rapid clearance of a clotting factor from the blood, or alter the clotting factor in such a way that the protein-antibody complex acquires a unique function. This review focuses on recent reports of autoantibodies directed against fibrinogen, prothrombin, factor V, factor VII, factor X, and von Willebrand factor, in which these various activities of autoantibodies are illustrated.