Reconnection is a universal plasma-physics process which has as one of its main characteristics the energization of plasma through the conversion of magnetic field energy. Interest in reconnection is multidisciplinary, and reconnection-associated phenomena are observed in astrophysical, solar system and laboratory plasmas. However, even though energy conversion is studied as a matter of practical interest and much fundamental research has been devoted to reconnection, this process has not been exploited very effectively in technological applications. To highlight the potential of reconnection for practical applications, we describe this process from a general point of view and present a theoretical model which is applicable to both non-relativistic and relativistic plasmas. Relativistic plasmas can be created under laboratory conditions, and the model predicts that such plasmas could be energized to very high energies. Experiments on reconnection may therefore lead to practical applications, such as hot plasma injectors in fusion reactors. Furthermore, we suggest that fusion reactors such as the tokamak can be modified so that plasma heating through reconnection is intrinsic to the device.