Spectrum sensing is the basic and essential mechanisms of Cognitive Radio (CR) to find the unused spectrum. This paper presents an overview of CR architecture, discusses the haracteristics and benefits of a CR. Energy detectionbased spectrum sensing has beenproposed and used widely because it doesnt require transmitted signal properties, channel information, or even the type of modulation. In this paper, a surveyof energy detector over Additive White Gaussian Noise (AWGN), different fading channels for spectrum sensing methodologies in cognitive radio is presented.Theoretical analysis of time domain energy detection and threshold setting is investigated. Cooperative spectrum sensing and a multiple antenna processing based energy detector receptions are also discussed.