Radio waves from a few kilohertz to millimeter-wave frequencies play a key role in modern wireless communications. The development over the last 120 years is traced with an emphasis on communication aspects and physical phenomena rather than theory. The early years were characterized by experiments with no theory and lack of knowledge of ionospheric propagation. High frequency (HF) propagation via the ionosphere at HF frequencies meant global communications for thousands of kilometers. Another natural medium is the atmosphere near the Earth’s surface, the troposphere, leading sometimes to anomalous phenomena, but it is also important for satellite signals near the horizon. Propagation over man-made structures like in an urban environment is covered by the simple Hata equations for first generation cellular systems. Higher generations must include delay information to accurately describe propagation, and the Hatalike equations may be extended into the millimeter frequency range. Indoor propagation may also be covered by a diffuse impulse response. Finally, the promise of increased spectral efficiency is given by multiple-input multiple-output (MIMO), if certain conditions of uncorrelated antenna signals are fulfilled.