The fundamental diurnal component of temperature, composition, and wind fields is discussed for the earth's thermosphere. The results are delineated in terms of energy sources in the lower atmosphere, mesosphere, and thermosphere, illustrating that EUV is the predominant excitation source above 130 km. Diurnal variations in the composition of H, He, O, O2, and Ar are analyzed in terms of effects from thermal expansion in diffusive equilibrium-normally assumed in empirical models-and diffusion associated with chemistry, wind circulation, and exospheric flow. Next to thermal expansion, wind-induced diffusion is the single most important process; it dominates the diurnal variations of He and prevails in the diurnal variations of O, O2, and Ar below 200 km. The wind effect is shown to eliminate isopyenic levels that develop in diffusive equilibrium solutions, and it shifts the phase of the lighter and heavier species to earlier and later local times, respectively. Exospheric flow dominates the diurnal variations of hydrogen, while winds are only important in the lower thermosphere where they increase the H amplitude by 50%. Dissociation of O2 and ion chemistry are of secondary importance for O but can significantly influence the diurnal variations of O2 such as by moving the phase to earlier local times. Comparison between theory and satellite data from Explorer 32, Ogo 6, San Marco, and AE-C shows substantial agreement, with the exception of O2, which requires an additional sink during daytime.