A description of the latitudinal, diurnal, seasonal, and solar-cycle variations of F-layer-produced scintillations, based on a comprehensive review of the literature, is given. Three distinct latitudinal regimes are discussed in regard to scintillation behavior and the quantitative utility of observations reported in the literature. A semiquantitative empirical model is suggested to describe the strength of scintillation-producing irregularities of F-layer electron density. Recent work suggests that more quantitative modeling might now be fruitful, but gaps in fully quantitative data exist. The prime needs are for observations at very high latitudes and for calibration of scintillation indices used at very low latitudes.