The engineering objective of high performance control using the tools of optimal control theory, robust control theory, and adaptive control theory is more achiev able now than ever before, and the need has never been greater. Of course, when we use the term high peiformance control we are thinking of achieving this in the real world with all its complexity, uncertainty and variability. Since we do not expect to always achieve our desires, a more complete title for this book could be "Towards High Performance Control." To illustrate our task, consider as an example a disk drive tracking system for a portable computer. The better the controller performance in the presence of eccen tricity uncertainties and external disturbances, such as vibrations when operated in a moving vehicle, the more tracks can be used on the disk and the more memory it has. Many systems today are control system limited and the quest is for high performance in the real world."