It was first pointed out by Hoyle et al. that quasars if they are indeed located at cosmological distances, must be characterized by an extraordinarily large radiation density : and therefore, though their optical and near infra-red spectra appear to be dominated by synchrotron radiation, any ultrarelativistic electron present must necessarily lose essentially all its energy by inverse Compton scattering. This would mean that, though quasars are emitting fantastic amounts of energy at optical frequencies, they must also be emitting many orders of magnitude more at X-ray and γ-ray frequencies. This paradox has been evaded by several specially constructed models (e.g. Rees and Sciama, Woltjer and Jukes); but the problem has not been removed in general.