Dynamic spectrum access (DSA) is a novel mechanism designed to increase the utilization of the radio spectrum. This paper presents a review of state-of-art with the references to the relevant works in the area of DSA. The most important DSA models and the main related challenges are introduced. Dynamic exclusive use model of DSA serves as the underlying platform for the implementation of spectrum trading in cognitive radio networks. Spectrum trading is a mechanism for flexible transfer of spectrum rights, which increases the revenue of a spectrum seller, and in parallel, the overall spectrum utilization as well. It is expected that the economy of the spectrum trading will play important role in the cognitive radio networks and thus, receive a huge attention currently. Therefore, the review of the relevant spectrum trading models is also referenced in this paper.