In the past two decades, there has been an explosive increase in demands for high-rate wireless communications featuring high reliability and low computational complexity. Therefore, many efforts have been made to understand the fundamental performance limits of wireless channels and the interlocking concepts of wireless communication techniques. In this thesis, we focus our study on some crucial wireless communication setups, namely multi-user, multiple-input multiple-output (MIMO) and cooperative communications in wireless networks. The purpose of the research is to develop communication strategies with capacity-achieving performance and low computational complexities. At the heart of this research lies a key technique which is referred to as _superposition coding_ in which the receiver sees the superposition of different levels of coded information. By exploring the concept of superposition coding, a unified view for these important communication setups are formed and the interplay of the concepts of these scenarios are demonstrated. For the setups under consideration, we first study their fundamental performance limits from information theoretic perspective. We learn that superposition coding schemes with interference cancellation based decoding are capacity-achieving for the majority of cases. Then, we develop practical superposition coding schemes featuring multi-level binary error coding and iterative soft interference cancellation and decoding. The developed approaches outrun existing communication schemes in either performance or computational complexities. For fading channels, we show that properly designed superposition coding scheme utilizes the degrees of freedom of the communication channel more efficiently while achieving the highest diversity gain. The advantageous features of the superposition coding technique will make it very attractive for practical implementations.