Random access algorithms provide the means to share network resources among users under distributed control. Traditional contention-based random access methods include pure, slotted, and reservation Aloha schemes, carrier sense multiple access (CSMA) and CSMA with collision avoidance schemes, multiple access with collision avoidance for wireless (MACAW) schemes, and so on [54, 199]. These existing medium access control (MAC) approaches do not consider channel state information (CSI) in the contention protocol. The drawback is illustrated in Figure 13.1. When MAC decides to transmit a frame, the channel may be in a deep fade. MAC may not transmit even though the channel is in a good state, which wastes channel resources. With opportunistic random access design, MAC frames will be transmitted when the wireless channel is good. Furthermore, when there are multiple users in the network, different users usually experience different channel states. Especially, users have peak channel conditions at different time periods. This effect can be called multi-user diversity and the aim is to exploit this multi-user diversity such that users with better or peak channel conditions win the contention for channel access. The more users in the network, the more likely there is to be a user with a good channel at any time. Therefore, the total throughput will increase with the number of users and consideration of channel states in the MAC design will significantly improve network performance. This chapter focuses on exploiting multi-user diversity in a single-cell network.