This book deals with electromagnetic resonance in the Earth-ionosphere waveguide and ELF propagation, by describing a general approach to the physical problems, electromagnetic ways to solve them, and properties of the solutions obtained. Attention is given to the discussion and interpretation of theoretical and experimental data and their links to global atmospheric conditions such as global thunderstorm activity, the lower ionosphere etc. Schumann resonance is related to stationary (or background) world-wide thunderstorm activity and simultaneously to global properties of the lower ionosphere. This resonance phenomenom is shown to be used as a global thermometer for global warming study. Transient events in the ELF/VLF range are another important aspect. These non-stationary signals are natural impulsed radio signals, which can be used to study cloud-to-ionosphere discharge (or mesospheric opitcal emissions) (red sprites, blue jets etc.). Clarification of the underlying physical ideas and their practical applications to this pioneering field are emphasized.