Cosmic rays are high-energy particles accelerated by nature to energies far beyond the reach of human-made accelerators. More than 100 years after the discovery of cosmic rays we still know little about their origin. The highest energy particles seem to originate from distant galaxies, but neither their sources are known nor are the most energetic sources in our galaxy, the Milky Way. More accurate measurements will be the key to understand the transition from galactic to extragalactic sources, and essential to find the most powerful cosmic accelerators in our galaxy and in the universe. To achieve the necessary accuracy different detection techniques have to be combined. In particular the radio technique for cosmic-ray air showers has become a competitive alternative. By complementing existing cosmic-ray observatories, the radio technique can boost the total accuracy to the level required for solving this long-standing mystery of astroparticle physics.