The book primarily focuses on cognitive radio systems. Considerable demands for new wireless services in the licensed and unlicensed frequency spectra have been observed due to the rapid growth of wireless communications sector. As many spectra remain under-utilized, so cognitive radio as a spectrum reuse method can be used in an efficient way for significant growth of communications resources. Cognitive radio has been widely popular in the field of academics and industry ever since it was first introduced in the later stages of last century. The main drawback is that its practical use has still not been successfully utilized. This book tries to describe some of the known problems in the field. The book has been written by intellectuals from around the world which includes subjects such as spectrum sensing fundamentals, cooperative sensing, spectrum management and interaction among users.