Recently cooperative relaying techniques have attracted much interest as a new way to combat fading in wireless networks. Basically, these techniques achieve spatial diversity gain and improve the link performance due to the transmission from multiple relays without the help of multiple antennas. In this case, the independent communication path created by the cooperative nodes can provide an opportunity to communicate better. So it has the potential to increase the rate and extent the network coverage and also reduce power consumption of transmitter nodes. In order to implement such cooperation among nodes, various approaches could be considered based on channel condition, complexity and distance between nodes. In this thesis, a cooperative system consisting of M relay nodes has been considered using DF-AF protocol in one way and two way relay mode. The main goal is to optimize power consumption of transmitter nodes so that the achievable rate is maximized and the outage probability is minimized. Because of the complexity in the calculation of outage probability, we have used some approximations to achieve lower bound for it. The outage probability and its lower bound are compared in simulation result. Also, it has been shown that the DF-AF protocol with optimum power allocation has better performance rather than other protocols. In the networks with M relays, orthogonal transmission is used. Therefore, the system throughput is limited by the number of relays. o, we aim to select the best relay among nodes to cooperate. It should be noted that if the relay selection has not been developed based on the appropriate metric, it will have the worst performance. In order to combat with this issue, two different selection approaches have been introduced and compared with each other in terms of outage probability. In simulation results it is shown that these approaches outperform the system with all relays. Key words: Cooperative Communication, DF-AF Protocol,Optimum Power Allocation, Relay Selection