Employing multiple antennas at the transmitter and receiver sides has been identified as the key enabler for high spectral efficiency in point-to-point communication, since it facilitates multiplexing of several data streams in space rather than in time/frequency. In this work, we are interested in achieving spatial multiplexing through efficient cooperative relaying schemes, even if both the transmit and receive antennas are distributed. To this end, we focus on wireless multiuser networks and aim at designing novel cooperative communication protocols, developing corresponding transmission and signal processing techniques, and optimizing the network performance.In the first part, we consider coherent multiuser amplify-and-forward (AF) relaying, where a set of source-destination (S-D) terminal pairs communicate concurrently over the same physical channel and a set of AF relay nodes assist the communication in a half-duplex scheme. We study various relay gain allocation schemes and system imperfections such as phase-noise and noisy channel estimates; investigate the necessary amount of channel information at relays and distributed computation of relay gains; and finally propose clustered relays networks.We shift our focus to decode-and-forward (DF) relaying in the second part, and address both multiuser one- and two-way multiple-input multiple-output (MIMO) relaying. First we consider two MIMO terminals exchanging information via a single MIMO relay node, and then, extend this scenario to the case of simultaneous MIMO communication of multiple S-D pairs. Therein, we investigate achievable rate regions, propose various relay transmit covariance optimizations, and consider channel estimation uncertainty.