In order to utilize the available spectrum more efficiently, cognitive radio systems have been proposed. Spectrum sharing in cognitive radios allows unlicensed users to access the licensed spectrum with primary licensed users. For achieving best spectral efficiency and non-interference with primary users, it is important to accurately detect the presence/absence of primary users. For this purpose, the solutions learned within the framework of decentralized detection in sensor networks have been considered. In this paper, we survey various approaches and results obtained for primary user detection in cognitive radio networks.