The theory for the diurnal tide in the atmosphere is reviewed and the appropriate Hough Functions (including those with negative equivalent depths) are described. The main thermal drives for the diurnal tide — insolation absorption by O3 and H2O — are described. The theory is then used to compute the atmosphere's response to the thermal drives. It is shown that the small amplitude of the diurnal surface pressure oscillation results from most of the thermal drive being used to activate trapped modes. The calculated winds are in reasonable agreement with observations of the diurnal wind oscillation from the troposphere to the upper mesosphere. In addition the tidal wind agrees strikingly with single time observations of the total wind in the upper-air. The temperature and vertical wind distributions and the upward flux of energy due to the diurnal tide are also calculated. The last item amounts to 6·8 ergs cm−2 sec−1 at the Equator which is about 2·5 times the downward solar flux in bands absorbable in the thermosphere. The average tidal flux over all latitudes is, however, considerably smaller.