This book chapter presents various relay selection policies based on spectral efficient techniques for the next generation mobile systems. Successive opportunistic relaying (SOR) that leverages the half-duplex constraint of conventional relays through concurrent transmissions is described, while proposing techniques to reduce the effect of inter-relay interference (IRI). An extension of SOR is defined for the case of out-of-band relaying, when additional spectrum bands are available and in networks with buffer-aided relays. Moreover, the use of full-duplex (FD) relays is outlined, in view of the effect of loop-interference (LI) from the relay's output to its input and, also, the power reduction is presented. As networks with multiple relays suffer from increased coordination overhead, a reduced Channel State Information (CSI) policy is proposed. For every policy, performance evaluation is provided in terms of outage probability, average throughput, power reduction and switching rate. Finally, open problems in spectral-efficient opportunistic relay selection policies are discussed.