  39 galaxies are now known, from follow-up of faint IRAS sources and from
submm observations of high redshift AGN, with far infrared luminosities >
10^{13} Lo. 13 of these, which have been found in 60 or 850 mu surveys, form an
important unbiased sub-sample. 12 have been found by comparison of 60 mu
surveys with quasar or radio-galaxy catalogues, or from infrared surveys with
colour selection biased towards AGN, while a further 14 have been found through
submm observations of known high redshift AGN. In this paper I argue, on the
basis of detailed modelling of the spectral energy distributions of
hyperluminous galaxies with accurate radiative transfer models, and from
evidence of high gas-mass in several cases, that the bulk of the emission from
these galaxies at rest-frame wavelengths >= 50 mu is due to star formation.
Even after correction for the effects of lensing, hyperluminous galaxies with
emission peaking at rest-frame wavelengths >= 50 mu are therefore undergoing
star-formation at rates > 1000 Mo/yr and are strong candidates for being
primeval galaxies, in the process of a major episode of star formation.
