The aim of this study is to focus on spectrum sensing in cognitive radio which is a recently introduced technology in order to increase the spectrum efficiency. Increasing efficiency of the spectrum usage is an urgent need as an intrinsic result of the rapidly increasing number of wireless users and also the conversion of voice oriented applications to multimedia applications. Static allocation of the frequency spectrum does not meet the needs of current wireless technology that is why dynamic spectrum usage is required for wireless networks. Cognitive radio is considered as a promising candidate to be employed in such systems as they are aware of their operating environments and can adjust their parameters. Cognitive radio can sense the spectrum and detect the idle frequency bands, thus secondary users can be allocated in those bands when primary users do not use those in order to avoid any interference to primary user by secondary user. There are several spectrum sensing techniques proposed in literature for cognitive radio based systems. In this thesis, energy detection and cyclostationary feature detection based spectrum sensing systems for cognitive radios with and without multiple antenna are examined in detail and comparative performance results are obtained in wireless communication channels.