In a conventional wireless cellular communication network, mobile stations (MSs) directly communicate with their associated base stations (BSs), which are the capacity bottleneck and limit the amount of traffic that can be supported in the system. The maximum transmission power of the stations limits the BS coverage. As new techniques are developed, MSs can have multiple air interfaces and communicate with each other either through in-band or out-of-band channels. Appropriately making use of the peer-to-peer communications may increase cellular network capacity, improve quality of service (QoS) to the users, and reduce communication costs. In this chapter we introduce several advanced networks that apply MS-to-MS communications in wireless cellular networks. In Sections 4.1 and 4.2, MSs not having their own traffic can relay traffic for other MSs. In Section 4.1 the MS-to-MS relaying uses out-of-band channel, while in Section 4.2 in-band and cooperative relaying is used for peer MSs to relay traffic for each other. In Section 4.3, MSs that communicate directly with each other form a cognitive radio network, which coexists with the cellular network and utilizes the radio resources of the cellular network through spectrum underlay. Power allocations are studied for each of these networks, and the network performance is analyzed.