Investigations of temperature inversions obtained from bathythermograph soundings in the Pacific revealed that inversions 0.50C or larger occur not only in polar regions but also in the subtropics and tropics. Such inversions are usually not associated with any static instabilities. The processes leading to the formation of the temperature inversions differ in different latitudes and under different flow conditions. In polar regions radiative and evaporative cooling appears to be the chief process; in subtropical latitudes inversions seem to be mostly due to differential advection of water of different origin. Inversions in the region of the jetlike countercurrents and undercurrents occur within the layer of strong velocity shear and may be due to turbulence. The largest temperature inversions, about 4°C, are found to occur near the Kuroshio-Oyashio boundary off northern Japan. Observations at fixed points indicate that the time variations of the intensity of the inversion and of the depth and vertical extent of the temperature maximum are of the same order of magnitude for hourly, daily, and monthly time scales. Several theoretical models are discussed.