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Relay communication systems have recently been gaining momentum as an alternative to cellular architectures because of the ability to provide cost-efficient high spectral efficiency communications. The goal of this thesis is to investigate opportunistic relay communication strategies in various network configurations. We derive the outage probability of opportunistic multi-hop multiple relay networks with nodes having single or multiple antennas. The challenge in the analysis of the outage probability are in incorporating realistic channel effects such as path loss, shadowing, and fast fading. With these effects, the outage probability depends on the energy transmitted, and also the shadow fading characteristics. We incorporate the channel dynamics in analyzing opportunistic relay selection schemes and determine the optimal selection period. As an extension of a conventional relay network, we explore a buffer-equipped relay network, where the network allows relays to delay transmission and transmit when the channel conditions are favorable. As a relay selection method, we suggest dual-timer relay selection (DTRS), which adopts the timer algorithm in reception and transmission relay selections. This algorithm reduces channel estimation overhead and solves the full-buffer problem of a reception relay and the empty-buffer problem of a transmission relay in buffer-equipped networks. We further consider a spatial reuse multi-hop relay network, and propose a full spatial reuse multi-hop (FSRM) relay communication scheme, which allows relays to transmit their data using every other time slot. With the FSRM scheme, the end-to-end rate reduction factor of a multi-hop relay communication is fixed at 1/2, regardless of the number of hops. We provide a comprehensive analysis of the outage probability of the proposed scheme for a directional antenna system and an omnidirectional antenna system.