The purpose is to review the coherent signal transmission experiments and theory that were associated with Columbia University's Hudson Laboratory. They are (1) continuous wave transmission (Berman, Clay, Guthrie, Frosch, Poss, Shaffer, Sherry, and Tolstoy); (2) reproducible signal transmissions (Clay, Parvulescu, and Williams); and (3) theory of coherent transmission in the ocean (Clav). The first experiments were made between a source suspended from a drifting ship and bottom-mounted hydrophones. These showed the fluctuations to be extremely small for the frequency band greater than 10−2 Hz. Bottom source-receiver transmissions showed phase fluctuations that were accountable to ocean surface waves. Reproducible signal transmissions using the matched signal technique were made by Parvulescu and Clay [“The Tongue of the Ocean,” Radio Eng. Electron. 29, 223–228 (1964)] and they suggested the concept of a correlation space for coherent transmission. Clay applied the adiabatic approximation to modes in the ocean and derived the magnitude and spatial correlation function of the coherent transmission [Rev. Geophys. 4, 475–507 (1966)].