Ambient noise is ubiquitous in the ocean. For many years, this noise has been considered the unwanted part of the acoustic signal. However, recent studies have shown that the noise itself contains valuable information about properties of the ocean, Earth, and atmosphere. For example, distant storms have been observed using measurements of low-frequency (0.1-Hz) noise that has propagated through the Earth’s core. Wind speed over the ocean has also been determined hundreds of kilometers away using noise measurements at coastal observing stations. At higher frequencies, surface wave noise due to breaking waves has been used to image the seabed and reveal details of the sub-bottom structure. In general, these techniques are based on cross-correlations of noise signals measured at different locations. These cross-correlations produce an estimate of the Green’s function (impulse response) between the two points which can be used to characterize the medium. With concerns over the impact of anthropogenic sound on the marine environment it is not surprising that remote sensing with naturally occurring noise has become a hot topic in acoustical oceanography. Essential components of noise processing will be described along with examples illustrating applications. [Work supported by ONR.]