Small scale ionospheric irregularities in the F-region can cause fluctuations in the amplitude, phase, and angle of arrival of VHF. UHF, and SHF signals traversing the ionosphere. Under some conditions, the power level fluctuations or scintillations at VHF and UHF may become severe with 12 dB signal level increases and fades in excess of 30 dB being observed. Current information about the probabilities of occurrence of severe fades is derived from a number of experiments using either radio star or satellite borne sources. The measurements are generally of signal level only and have been used to calculate scintillation indices to characterize scintillation intensity. An examination of the global distribution of scintillation indices show that scintillations are of importance to communication system performance primarily in the auroral and polar regions and at night near the geomagnetic equator.