The results of 15 years of plasma resonance observations from rocket- and satellite-borne topside sounders are summarized. The major resonances are observed at the electron plasma frequency ƒN, the electron cyclotron frequency ƒH, the upper hybrid frequency ƒT = (ƒ2N+ƒ2H)1/2, the harmonics 2ƒT and nƒH with n > 1, and two series of resonances that appear between nƒH∶ the ƒQn resonances observed above ƒT (at the maximum frequency of the Bernstein modes) and the ƒDn resonances observed below ƒT (called the diffuse resonances). Most of the resonances can be interpreted in terms of the reception of longitudinal plasma waves stimulated by the sounder pulse. In some cases these waves, which travel with low group velocity (of the order of the electron thermal velocity or less) are received after being reflected in the nearby medium (within a distance of several kilometers). The reflections are due to the extreme sensitivity of the dispersion curves near the resonant frequencies to small changes in electron density (for the ƒN and ƒT resonances) or to small changes in magnetic field strength (for the lower-order nƒH resonances). In other cases (ƒQn resonances and the higher-order nƒH resonances) the signal reception is due to the matching of a component of the wave group velocity to the satellite velocity rather than to a wave reflection process. The ƒDn resonances involve a sounder-stimulated electron temperature anisotropy leading to the Harris instability of the Bernstein modes and a nonlinear coupling between these wave-modes. Since the interpretation of all of the resonances depends on warm plasma theory, the observations can be used to obtain the electron temperature T⊥ and T∥ (corresponding to electron motions perpendicular and parallel to the ambient magnetic field direction, respectively) as well as the electron density and the magnetic field intensity. These measurements pertain to a large volume (hundreds of meters to kilometers) around the antenna and thus are not seriously affected by spacecraft/plasma interactions. The results of the investigations of artificially stimulated plasma waves by sounder pulses have been applied to the interpretation of naturally occurring wave phenomena such as the (n+1/2)ƒH magnetospheric VLF emissions, the terrestrial kilometric radiation, and the Jovian decametric radiation. Among the major unexplained plasma resonance phenomena are the resonances observed at ƒH and 2ƒT, and the numerous ion effects associated with the electron plasma resonances. The advent of the Space Shuttle with subsatellites will enable active-controlled experiments to be conducted with highly flexible radio-frequency instrumentation in order to investigate problems of interest to many geophysical and astrophysical phenomena such as wave mode coupling and the evolution of nonlinear and plasma wave instability processes.