The structure and variability of the middle atmosphere of Venus (60 to 140 km) were studied from the Pioneer Venus orbiter by using an infrared remote sensing instrument developed from those on terrestrial weather satellites. The wavelengths observed were selected to allow the vertical temperature profile, the albedo, the cloud opacity profile, and the far infrared opacity due to water vapor to be inferred from the data. The measured temperature field has been used to model the dynamics of the region, and the thermal and solar fluxes have been used to compute the planetary radiation budget. The results for the diurnal variation of temperature at a given height show fairly small amplitudes up to an altitude of about 95 km, above which the day to night contrast increases rapidly with height. At the equator the dependence of temperature in the stratosphere (65 to 95 km) on solar longitude is dominated by a wave number 2 solar tide with an amplitude of about 10 K. Transient features including traveling waves are also present on a wide range of scales. The equator to pole gradients are larger than expected, and the stratosphere is typically 15 to 20 K warmer at the pole than at the equator. Nightside temperatures in the mesosphere (95 to 140 km) are generallly low except for a local maximum near the antisolar point, and breakdown of local thermodynamic equilibrium is evident above about 120 km. The winds forced by the measured temperature field in a diagnostic circulation model show the '4-day' zonal winds decreasing rapidly with height above the clouds and becoming very small by 80 or 90 km. altitude. The mean meridional component reverses at about the same altitude and pole-to-equator winds as high as 100 m s-1 are produced above 100 km. The most significant discovery concerning the cloud morphology is a dramatic 'dipole' structure, consisting of two clearings in the cloud at locations straddling the pole and rotating around it every 2.7 days. The clearings are thought to be evidence for subsidence of the atmosphere at the center of a polar vortex. The absence of corresponding evidence for descending motions elsewhere suggests that a single large circulation cell may fill the northern hemisphere at levels near the cloud tops. A crescent-shaped 'collar' region, consisting of anomalous and variable temperature and cloud structure, surrounds the pole at about 70 ¿N and rises perhaps 15 km above the mean cloud top elevation; it has a solar-fixed component and sometimes contains spiral streaks. This feature, and the double vortex eye, are large, persistent deviations from the mean circulation due to planetary-scale waves of unknown origin. No explanation is offered at present for the dominance of wave number 2 structures at equatorial and polar latitudes, while the mid-latitudes are dominated by a wave number 1 feature (the polar collar). A thin, ubiquitous haze is found covering the northern hemisphere, including the polar features. The far-infrared opacity of the atmosphere is greater in the afternoon than at any other local time and also tends to increase at high latitudes; the most likely cause is a small but variable amount of water vapor. The angular dependence of the intensity of scattered sunlight fits a cloud model consisting of 1-&mgr;m droplets. The observed angular and planetographic distribution of total reflected solar energy and emitted thermal flux, when integrated over the northern hemisphere of Venus, are consistent with radiative balance to within the accuracy of a preliminary calculation.