L1 Adaptive Control Theory presents a new and efficient way of developing control laws for highly uncertain or rapidly changing environments. The key feature of this new theory is the decoupling of adaptation from robustness: the architectures of L1 adaptive control theory have guaranteed transient performance and robustness in the presence of fast adaptation. The authors cover detailed proofs of the main results and also present the results of flight tests which used this theory. The material is organized into six chapters and concludes with an appendix that summarizes the mathematical results used to support the proofs. This book is intended for graduate students; researchers; and aerospace, mechanical, chemical, industrial and electrical engineers interested in pursuing new directions in research and developing technology at reduced costs.