This book contains the spectrum sensing issue in cognitive radio. In order to utilize the spectrum efficiently, the role of spectrum sensing is essential in cognitive radio networks. The transmitter detection based techniques, energy detection, cyclostationary feature detection, and matched filter detections are most commonly used for the spectrum sensing. However, detection performance in practice is often compromised with multipath fading, shadowing and receiver uncertainty issues. To mitigate the impact of these issues, cooperative spectrum sensing has been shown to be an effective method to improve the detection performance by exploiting spatial diversity. The main idea of cooperative sensing is to enhance the sensing performance by exploiting the spatial diversity in the observations of spatially located CR users. By cooperation, CR users could share their sensing information for making a combined decision more accurate than the individual decisions. Thus the Cooperative sensing can formulate excellent use of network assets and make the network smooth.