The scarcity of energy and bandwidth are the two fundamental resources for communications. It imposes severe limitations on the development of communications networks in terms of bandwidth, capacity and performance. To utilize the scarce resources intelligently and efficiently, new technologies are developed such as cooperative communication and cognitive spectrum sharing. Both the technologies have shown great potential for enhancing the performance of wireless networks and meeting the demands of future wireless applications. The relay node concept in CRAHNs has been used in this paper. The co-operations between multiple relay nodes increase the throughput and reduce the loss of bandwidth efficiency. The various methods of selecting the relay nodes are discussed in this paper.