Fusion provides the promise of a relatively clean and inexhaustible energy source that could allow world-wide development at a reasonable price. The Tokamak concept is by far the best option currently available to operate as a future fusion reactor. After years of experimentation with small size devices, next-step tokamaks will be of large enough dimensions as to be affected by issues that could be neglected in the past. One of such issues is that of runaway electrons, which may also play an important role in other types of plasmas. Runaway electrons are generated during tokamak operation whenever a sufficiently large toroidal electric field is present. Acceleration in this electric field can then overcome the collisional drag allowing these electrons to reach energies of tens and even hundreds of MeV. One such situation is usually encountered whenever the discharge violently ends in a disruption. The poloidal magnetic flux decreases then very quickly giving rise to a strong inductive toroidal electric field. Runaway electrons generated in this process have been recently conjectured to be a potential hazard for future large-scale reactor operation.