This monograph combines the commutant lifting theorem for operator theory and the state space method from system theory to provide a unified approach for solving both stationary and nonstationary interpolation problems with norm constraints. Included are the operator-valued versions of tangential Nevanlinna-Pick problem, the Hermite-Fejer problem, the Nehari problem, the Sarason problem, and the two-sided Nudelman problem, adn their nonstationary analogues. The main results concern the existance of solutions, the explicit construction of the central solutions in state space form, the maximum...