  Extensive air showers still are our only access to the highest-energy
particles in the universe, namely cosmic-ray nuclei with energies up to several
100 EeV. Studying open questions in cosmic-ray physics, like their yet unknown
origin requires the reconstruction of the energy and mass of the primary
particles from the air-shower measurements. Great progress has been achieved
lately in the development of the radio detection technique for this purpose.
There now is a consistent picture of the mechanisms behind the radio emission,
which is in agreement with measurements. Several second-generation, digital
antenna arrays are operating in different parts of the world not only aiming at
the further development of the technique, but also contributing to cosmic-ray
physics at energies above 100 PeV. Recently it has been demonstrated
experimentally that radio detection can compete in precision with established
techniques for air showers, like the measurement of secondary particles on
ground, or fluorescence and Cherenkov light emitted by air showers.
Consequently, cosmic-ray observatories can benefit from radio extensions to
maximize their total measurement accuracy.
