Cosmic rays with high energies induce extensive air showers when they impinge on the atmosphere of the Earth. A large number of the particles in air showers are electrons and positrons. They interact with the magnetic field of the Earth and emit radio waves. Radio waves with frequencies of tens of Megahertz are used as a tool to measure the properties of cosmic rays, such as their energy, particle type (mass) and arrival direction. The basic mechanisms of the emission process, actual detectors, and recent results will be discussed.