Spectrum sensing is a base for the performance of all functions performed by the Cognitive Radio (CR). Cognitive radio recognizes the unused spectrum and shares it to secondary users (SUs) without creating harmful interference to primary users (E.g. Cellular Networks, TV). This book discusses various Spectrum Sensing techniques and compares their performances and also observed the performance of Single user and Global decision using Modified Deflection Coefficient (MDC) method. Extensive study of cooperative Spectrum Sensing and optimal cooperative sensing is done and results are presented.