The observational and theoretical work on the low-frequency electromagnetic oscillations of the earth-ionosphere cavity, known as the Schumann resonances, is reviewed. Additional theoretical considerations are introduced to allow the models to incorporate the asymmetries of the actual cavity. These models are used to explain the observed variations of the power and of the apparent frequency of the resonant modes. The theoretical studies show that the structure of the low D or C region of the ionosphere is critical to these resonances, and the observed frequencies must rule out certain proposed ionospheric models. The perturbations of the cavity are also studied, and comparisons are made between different types of perturbations. The resonance data from July 9, 1962, are used to confirm the tremendous extent of the ionospheric perturbations caused by the Starfish high-altitude nuclear explosion. The possible effects of natural perturbations are also examined.