Advances in Control Systems: Theory and Applications, Volume 5 provides information pertinent to the significant progress in the field of control and systems theory and applications. This book presents the problem of the optimal control of a system. Organized into six chapters, this volume begins with an overview of the fundamental conditions in the calculus of variations that are basic to the optimal control problem. This text then examines one of the basic problems in control and systems theory in general. Other chapters consider a number of rather basic results in optimal nonlinear filtering and describe the characteristic function of the state of vector of a nonlinear system. This book discusses as well a significant application area of control and systems theory, which is the optimal control of nuclear reactors. The final chapter deals with optimal control with bounds on the state variables. This book is a valuable resource for practicing engineers.