  Nearly 50 years ago, the first radio signals from cosmic ray air showers were
detected. After many successful studies, however, research ceased not even 10
years later. Only a decade ago, the field was revived with the application of
powerful digital signal processing techniques. Since then, the detection
technique has matured, and we are now in a phase of transition from small-scale
experiments accessing energies below 1018 eV to experiments with a reach for
energies beyond 1019 eV. We have demonstrated that air shower radio signals
carry information on both the energy and the mass of the primary particle, and
current experiments are in the process of quantifying the precision with which
this information can be accessed. All of this rests on a solid understanding of
the radio emission processes which can be interpreted as a coherent
superposition of geomagnetic emission, Askaryan charge-excess radiation, and
Cherenkov-like coherence effects arising in the density gradient of the
atmosphere. In this article, I highlight the "state of the art" of radio
detection of cosmic rays and briefly discuss its perspectives for the next few
years.
