A large fraction of the energy released during solar flares is carried by accelerated electrons and ions. At present the most direct diagnostics of accelerated electrons are the radio emission produced from the microwave to the sub-millimeter domains by gyrosynchrotron emission and the hard X-ray/gamma-ray continuum from electron bremsstrahlung. These diagnostics have brought valuable information on the electron energetics, but the characteristics of relativistic electrons are still poorly known, although they provide the most stringent constraints for acceleration models. This is, at least partly, because radio diagnostics were usually obtained below 100 GHz and gamma-ray measurements were restricted to energies below 100 MeV. Recent radio observations in the 200–400 GHz range have revealed a new spectral component, the origin of which is not yet understood. Observations at higher frequencies are necessary in order to discriminate between different possibilities. Although solar observations in the far infrared domain have not yet been performed, this spectral region contains key information on both relativistic electrons and the response of the low atmosphere to energy deposition from the flare. The combination of observations obtained in the far infrared continuum (2–10 THz) with measurements of high energy gamma-rays (up to 500–600 MeV) will not suffer from the limitations of the present diagnostics and thus provide new constraints on acceleration and energy transport processes.