This book provides a comprehensive view of current developments in the theory of recursive reduced-order methods for singularly perturbed and weakly coupled linear control systems. The recursive methods offer several advantages: high accuracy can easily be achieved at low cost, parallel processing can be used, results are obtained under mild assumptions, and software and hardware implementation of the control algorithms is highly simplified. The book emphasizes mathematical developments as well as their application to solving practical problems. A number of real world examples demonstrating the usefulness of the theory are included.