Since its inception in the early 1980s, H( optimization theory has become the control methodology of choice in robust feedback analysis and design. The purpose of this monograph is to present a self-contained operator theoretic approach to the H( control for disturbed parameter systems, that is, systems which admit infinite dimensional state spaces. Such systems arise for problems modelled by partial differential equations or which have time delays. Besides elucidating the mathematics of H( control, treatment is given to its physical and engineering underpinnings. The techniques given in the book are illustrated by two benchmark problems: an unstable system with a time delay which comes from the control of the X-29, and the control of a Euler-Bernoulli flexible beam with Kelvin-Voigt damping.