The homotopy type of a closed simply connected 4-manifold is determined by the intersection form. The homotopy classes of maps between two such manifolds, however, do not coincide with the algebraic morphisms between intersection forms. Therefore the problem arises of computing the homotopy classes of maps algebraically and determining the law of composition for such maps. This problem is solved in the book by introducing new algebraic models of a 4-manifold. The book has been written to appeal to both established researchers in the field and graduate students interested in topology and algebra. There are many references to the literature for those interested in further reading.