Rapid fluctuations in the amplitude and phase of electromagnetic or acoustic waves that have propagated through a medium containing fluctuations in refractive index, such as the atmosphere. The most common example of optical scintillation is the “twinkling” of stars observed through the atmosphere. Scintillation arises as a result of random angular scattering produced by refractive index fluctuations. For electromagnetic wave propagation, these result from fluctuations in temperature and, especially at far infrared and radio frequencies, humidity. Scintillation in acoustic wave propagation arises from velocity and temperature fluctuations. Fluctuations in the amplitude of different frequency components in the spectrum of an object can give rise to apparent changes in its color (chromatic scintillation); an example is the random red and blue twinkling of bright stars near the horizon. Scintillation statistics have been used to study turbulence in regions ranging from the planetary boundary layer to the ionosphere, as well as interplanetary and interstellar space. Scintillation is important for astronomical imaging, optical and radio communications, laser and acoustical propagation, active and passive remote sensing, and the performance of the Global Positioning System.