The observational evidence for long-range propagation of high-frequency radio signals began to accumulate in 1926 when transatlantic communication links were established. In 1927, signals were detected that had propagated over large distances or circled the earth with little attenuation. Long-range and round-the-world (RTW) signals showed preferred periods of occurrence. Relatively high angles of incidence (20° relative to the horizontal) of such signals at the receiver paired with low attenuation ruled out conventional multihop propagation. An ionospheric refractive index boundary was postulated along which the signal propagates in a glancing incidence low-loss mode. The transmission path of observed long-delayed signals, originating from artificial earth satellites, was found to be located in the twilight zone. Experiments that used ground-based transmissions confirmed the role of ionospheric absorption and tilts for the occurrence of long-range and RTW signals. Detailed explanations for ionospheric ducting were provided by using several theoretical concepts and analyses. The μr (refractive index times geocentric distance) diagram was used for the study of ray propagation in a spherically stratified multilayered model ionosphere depicting characteristics of the ground-detached glancing incidence mode. The potential well analysis was applied to an inhomogeneous slowly varying ionosphere. Numerical ray-tracing computations were made by using model ionospheres with horizontal gradients. Ionospheric models were used to predict global properties of ionospheric wave channels. The role of scattering from natural and artificially induced ionospheric irregularities in long-range ionospheric ducting was investigated.