The ionospheres of the giant planets Jupiter, Saturn, Uranus, and Neptune are reviewed in light of Pioneer, Voyager, Galileo, and groundbased infrared observations. A major focus of recent research has been on the interpretation of Jovian auroral emissions at infrared, ultraviolet, and X-ray wavelengths. Analysis of these emissions has provided valuable clues to the still poorly understood nature of the magnetosphere–ionosphere interaction at Jupiter. Models of the ionospheres of the outer planets incorporating a neutral thermal structure derived from spacecraft measurements and the effects of vibrationally excited H2 molecules, vertical plasma drift, and particle precipitation have been developed and reproduce reasonably well the ionospheric structures determined from the radio occultation experiments. Other recently reported research include the effect of upward propagating gravity waves on the Jovian ionosphere, a one-dimensional, time-dependent model of Saturn's ionosphere, and a photochemical model of Neptune's upper atmosphere. These studies suggest that the introduction of meteoric ions and gravity waves may impact the hydrocarbon ion chemistry of the lower ionospheric regions of the outer planets. Finally, we report the development of three-dimensional general circulation models, which are now able to investigate the physical processes of the outer planets' thermospheres self-consistently with the global thermal structure and global distribution of the neutral and ions.