Opportunistic spectrum access has become a high priority research area in the past few years. This paper is the fact that the limited spectrum available is currently being utilized in an inefficient way. The complete wireless spectrum is assigned and reserved, but not necessarily being used. At the same time, the demand for innovation in wireless technology is growing. Since there is no room in the wireless spectrum to allocate significant frequency bands for future wireless technologies, the only recourse is to increase utilization of the spectrum. To achieve this, the authors must find a way to share the spectrum. This paper surveys the problem of the dynamic spectrum access in cognitive radio networks.