Cognitive Radio networks provide advancement to the wireless generations. Due to the fixed spectrum allocation policy, a wider portion of the spectrum is underutilized in many areas. Spectrum sensing adds intelligence to Cognitive Radios that dynamically identifies the spectrum holes. Many spectrum sensing algorithms are proposed in literature to sense the spectrum holes in a noisy environment. But, there are several challenges that are to be considered in simulating the algorithms in a real-time channel. In this paper, a review of various issues of spectrum sensing is discussed.