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Differential phase-shift keying (DPSK) has attracted significant attentions in research and development during the last several years. An overview of DPSK for high spectral efficiency optical transmission is presented in this paper. The advantages of DPSK in terms of receiver sensitivity and tolerance to fiber nonlinearity will be discussed in detail. A simplified method for estimating the performance of phase-shift keying in numerical simulations is explained. Results of experimental and numerical investigations of several phase shift keying formats, including polarization division multiplexing and multilevel encoding, will be reviewed. Finally, methods for further enhancing performance of phase-shift keying in long-haul transmissions, specifically the compensation of nonlinear phase jitter, are presented.