The technique of cooperative communications is finding its way in the next generations of many wireless communication applications. Due to the distributed nature of cooperative networks, acquiring fading channels information for coherent detection is more challenging than in the traditional point-to-point communications. To bypass the requirement of channel information, differential modulation together with non-coherent detection can be deployed. This thesis is concerned with various issues related to differential modulation and non-coherent detection in cooperative networks. Specifically, the thesis examines the behavior and robustness of non-coherent detection in mobile environments (i.e., time-varying channels). The amount of channel variation is related to the normalized Doppler shift which is a function of user’s mobility. The Doppler shift is used to distinguish between slow time-varying (slow-fading) and rapid time-varying (fast-fading) channels. The performance of several important relay topologies, including single-branch and multi-branch dual-hop relaying with/without a direct link that employ amplify-and-forward relaying and two-symbol non-coherent detection, is analyzed. For this purpose, a time-series model is developed for characterizing the time-varying nature of the cascaded channel encountered in amplify-and-forward relaying. Also, for single-branch and multi-branch dual-hop relaying without a direct link, multiple-symbol differential detection is developed. First, for a single-branch dual-hop relaying without a direct link, the performance of two-symbol differential detection in time-varying Rayleigh fading channels is evaluated. It is seen that the performance degrades in rapid time-varying channels. Then, a multiple-symbol differential detection is developed and analyzed to improve the system performance in fast-fading channels. Next, a multi-branch dual-hop relaying with a direct link is considered. The performance of this relay topology using a linear combining method and two-symbol differential detection is examined in time-varying Rayleigh fading channels. New combining weights are proposed and shown to improve the system performance in fast-fading channels. The performance of the simpler selection combining at the destination is also investigated in general time-varying channels. It is illustrated that the selection combining method performs very close to that of the linear combining method. Finally, differential distributed space-time coding is studied for a multi-branch dual-hop relaying network without a direct link. The performance of this network using two-symbol differential detection in terms of diversity over time-varying channels is evaluated. It is seen that the achieved diversity is severely affected by the channel variation. Moreover, a multiple-symbol differential detection is designed to improve the performance of the differential distributed space-time coding in fast-fading channels.