Motivated by the inefficient usage of the licensed spectrum across many frequency bands, cognitive radio (CR) was proposed to promote the spectrum utilization by opportunistically exploiting the existence of spectrum holes. Under this new paradigm, cognitive (unlicensed) user is allowed unobtrusive access to a channel exhibiting negligible primary (licensed) user activity. Cooperative communication is increasingly regarded as a way to address hidden terminal issues and to improve spatial diversity for the same spectrum frequency band. In this paper, we investigate two scenarios of cooperation: cooperative transmission of primary traffic by cognitive users (cognitive relaying) and cooperative transmission between cognitive nodes. The first scenario focuses on one cognitive transmitter which has the option to relay traffic of the primary assuming that the primary is oblivious to its presence and thus exclude the possibility of spectrum leasing. While the latter investigates a spectrum-rich node that is selected as the relay node to improve the performance between the source and the destination. To demonstrate the feasibility and performance of cooperative relay for cognitive radio, numerical results based on information theory approach are provided in order to discuss the advantages and limits of the two strategies.