The theory of dual algebras (ultra weakly closed algebras of operators on Hilbert space) has made tremendous progress since 1978, when Scott Brown originated some of the main ideas to solve the invariant subspace problem for subnormal operators. The impetus for much of this progress has come from the authors of the present book, who, in a sequence of papers, have added several new ideas concerning the solution of systems of simultaneous equations in the predual of a dual algebra, thereby developing a dilation theory and contributing substantially to the theories of invariant subspaces and reflexivity. In addition to containing the major results of the theory as presented in earlier papers by the authors and other mathematicians, this important study presents much material not previously available elsewhere. Accessible to graduate students having knowledge of a first course in operator theory, this excellent book will be of interest to all researchers in the field.