The objectives of spectrum sensing are twofold: to start with, CR users ought not cause unsafe interference to PUs by either switching to an available band or limiting its interference with PUs at an acceptable level and, second, CR users ought to efficiently identify and exploit the spectrum holes for required throughput and quality-of service (QoS). probability of false alert, which denotes the probability of a CR user declaring that a PU is present when the spectrum is in reality free, and probability of detection, which denotes the probability of a CR user declaring that a PU is present when the spectrum is indeed occupied by the PU. In this paper review the different methods of spectrum sensing.