This review paper summarizes present knowledge about turbulence in the free atmosphere. Data from various sources allow the construction of an average spectrum of atmospheric motions which covers almost the entire range of periods from 1 second to 1 year. It is shown that the micrometeorological maximum, which probably exists in cases of moderate and severe CAT (clear air turbulence), is not separated from synoptic scale motions by a deep mesoscale gap. Theoretical considerations and observations in the laboratory, the ocean, and the free atmosphere suggest that CAT is a manifestation of the Kelvin-Helmholtz instability. This implies that for better understanding of CAT, for forecasting, and for planning of air-borne measurements, isentropic analysis of the meteorological situation is advisable. Sophisticated aircraft instrumentation is now beginning to yield data sufficiently accurate and detailed to make feasible the use of the powerful tool of energy budget analysis for CAT studies. Recent experiments in the atmosphere have not clarified the problem of the existence of the buoyant subrange and the corresponding behavior of the spectrum. Development of a high-resolution vertical sounding technique permits the measurement of the spectra of the horizontal winds in the vertical direction, which may make possible the prediction of microturbulence features from vertical wind profile analysis. Temperature spectra in the free atmosphere are discussed, and the values of the temperature dissipation rate and of the structure function coefficient are calculated for CAT of several different intensities.