This book is intended to serve as an introduction to the subject of adaptive array sensor systems whose principal purpose is to enhance the detection and reception of certain desired signals. Array sensor systems have well-known advantages for providing flexible, rapidly configurable, beamforming and null-steering patterns. The advantages of array sensor systems are becoming more important, and this technology has found applications in the fields of communications, radar, sonar, radio astronomy, seismology, and ultrasonics. The growing importance of adaptive array systems is directly related to the widespread availability of compact, inexpensive digital computers that make it possible to exploit certain well-known theoretical results from signal processing and control theory to provide the critical self-adjusting capability that forms the heart of the adaptive structure. The field of adaptive array sensor systems is now a maturing technology, and with applications of these systems growing more and more numerous there is a wealth of widely scattered literature available on various aspects of such systems. There are even a few textbooks that briefly treat certain aspects of adaptive array systems, but until now there have been no books devoted entirely to presenting an integrated treatment of such systems that provide the reader with the perspective to organize the available literature into easily understood parts. The decision to write this book was made as a result of this lack. The primary emphasis of the book is to cover those principles and techniques that are of fundamental importance in modern adaptive array systems. Most of the contents are derived from readily availablesources in the literature, although a certain amount of original material has been included.