In complicated settings, time reversal (TR) takes use of spatial variety to accomplish spatial and temporal concentration. The TR approach has been successfully used to phase-coherent acoustic communications in time-varying multipath ocean settings as an alternative to traditional adaptive multichannel equalization during the past decade. Intersymbol interference (ISI) is reduced by temporal focusing (pulse compression), and residual ISI is removed by single-channel equalization, resulting in near-optimal performance in principle. The spatial focusing feature allows multiuser or multiple-input–multiple-output (MIMO) communications to take place without the need of time, frequency, or code division multiplexing, and an adaptive TR technique may further minimize crosstalk between users or multiple transmitters. Using a block-based method with channel updates, TR communications may simply be expanded to time-varying channels. This article discusses TR communications in shallow and deep water, as well as current developments such as bidirectional equalization, multiuser communications with mobile users, and communication with a glider acting as a mobile gateway