Recently, there has been much interest in modulation§techniques to achieve transmit§diversity motivated by the increased capacity of§multiple-input multiple- output (MIMO)§channels. It is affordable to equip base stations§with more than one antenna, but it is§difficult to equip the small mobile units with more§than one antenna. In such a case,§transmit diversity can only be achieved through user§cooperation leading to what is known§as cooperative diversity. Cooperation provides a new§dimension over which higher diversity§orders can be achieved.§§In this thesis, we consider the design of protocols§that allow several terminals to cooperate§via forwarding each others' data, which can increase§the system reliability by achieving§spatial cooperative diversity. We consider the§problem of how to achieve and where to§exploit diversity in cooperative networks?