New material, improvements and recent advances have been added to this second revised edition. The volume examines the general theory for constant coefficient operators, elliptic operators, the L 2 theory for operators bounded from below, and self-adjoint operators. A comprehensive theory for second order operators is given, and applied to quantum mechanical systems of particles. Since many of the topics treated here are of interest to chemists, engineers, mathematicians and physicists, the volume begins with background and reference material.