This paper gives a brief survey of the state of knowledge in ocean acoustics circa 1948 and reviews the advances made during the past two decades. Twenty years ago the thermostructure of the ocean was known in moderate detail. while the character of the boundaries could be specified less precisely. A principal deficiency was in the ability to handle a mathematical model of the ocean which would give a prediction of the acoustic field in detail. While much remains unknown today, the past 20 years have seen a considerable accumulation of knowledge concerning the geographic and seasonal variations of the medium. Reasonably good specifications of the ocean boundaries are now available. The acoustic absorption coefficient in the ocean is known imperfectly, and the ability to specify the thermo microstructure is poor. Nonetheless, the intensive efforts of many investigators over the last 20 years have yielded a computational capability which now allows a general specification of propagation in relatively great detail.