It is suggested that blue compact galaxies may be galaxies of normal age but, if so, they must have haloes much more massive than the observed compact core. If such a galaxy has at some stage in its evolution been free of gas, further star formation from gas expelled by stars is only likely to occur if the gas accumulates in a region in which the total mass of gas is at least comparable with the mass of stars. This could then lead to the appearance of a blue compact core. In addition, as most of the mass lost by supernovae escapes from such a galaxy, the observed deficiency of heavy elements in at least some blue compacts can perhaps be understood. If this model is not capable of explaining the properties of the blue compacts, it seems likely that they are genuinely young galaxies.