When NRL began operations in 1923, it had two research divisions: Sound (renamed Acoustics in 1968), and Radio. The Sound Division was quite small prior to World War II, with fewer than ten researchers. However, the accomplishments of Sound Division were of immense importance to the U.S. Navy. In the 1920s and 1930s, Sound Division researchers conducted investigations to develop sonar equipment that was installed on Navy vessels at the outset of World War II that helped significantly to turn the tide in favor of the Allied Forces during undersea warfare operations. In the 1950s, research was begun to evaluate the potential for lower frequency sonar systems with improved range capability, beginning with systems for 10 kHz, then 5 kHz, then 1 kHz and lower frequencies. In the 1950s and 1960s, important new research was begun on signal processing techniques, the scattering of sound from the ocean boundaries and from underwater objects with simple shapes, the speed of sound in water, and on models to understand how sound propagates in the ocean. In the 1970s there was considerable research on long-range acoustic propagation and scattering with an increased emphasis on physical acoustics, as well as initial research on fiber-optic interferometric acoustic sensors. The 1980s included much research in Arctic acoustics and was the last decade with primary emphasis on deep ocean acoustics. In the 1990s the emphasis shifted toward research in the littoral oceans. Also in the 1990s, the acoustics researchers from the Mississippi-based Naval Ocean Research and Development Activity (NORDA)/Naval Ocean and Atmospheric Research Laboratory (NOARL) became part of the Acoustics Division. In the 2000s and beyond the Acoustics Division has maintained a vigorous and diverse program of research in ocean acoustics with emphasis on Navy applications and extensions to new areas such as nanoscience and quantum acoustics.