Book excerpt: In recent years, it was realized that Radio Cognitive Systems seems to be a real need in the near future evolution,especially because of the severe spectrum regulations and channel congestion problems. Moreover high data rates become more and more necessary for normal professional needs or at home based users. Cognitive radio systems were first defined in the late of 1990s. The idea is to integrate a new concept of channel environmental sensing, followed by a knowledge based standard learning, which in return helps to identify free channel spectrum slots. These slots can be used by a third party user in a way to take advantage of the unused spectrum.This type of communication provides dynamic opportunistic algorithm for spectrum access. The new concept has to be adaptive and based on a reconfigurable hardware with a real time processing. Such cognitive radio systems can be also used in indoor wireless communications, where more and more wireless peripherals and tools are used in a limited space, and once again data packet congestion is a real problem. This book is written by specialists working in the field of telecommunication and signal processing. Various aspects of the radio cognitive systems are discussed with some applications and implementations such as software defined radios. The book is composed of seven chapters; as each chapter is written in a self-contained manner, the reader can use the book without any restrictive ordering of the chapters. At the end of the chapters there are valuable references that provide in depth coverage of the application...