In this book, the authors present the newly developed theory of non-harmonic Fourier series and its applications to the control of distributed parameter systems, and they extend the theory to include vector exponential series. The first part of the book presents the modern theory of exponentials, using an operator theory approach. The second extends and upgrades the method of moments--one of the most powerful tools in the flourishing theory of the control of distributed parameter systems. They then go on to discuss the controllability of systems described by parabolic and hyperbolic PDE's for internal, boundary, initial, and pointwise control, and consider typical applications to optimal control problems. Researchers in control theory, operator theory, functional analysis, and partial differential equations will find much to interest them in this treatise.