Model atmospheres with temperatures in the range 10 000 Teff 25 000 0K and with gravities 6 log (g) < 9 have been constructed for various helium abundances with a view to understanding the spectra of the hotter white dwarfs. The roles of accretion, convection, circulation, gravitational diffusion and magnetic fields are also examined. It is shown that the DB stars are confined to the temperature range 15 0000 < Teff < 18 0000 in which convection also becomes important in the outer layers of a helium rich star. The DA stars seem to avoid this temperature range. The hydrogen and metal abundances in DB atmospheres are shown to be reduced by factors io5 and io3 respectively compared to solar values. Possible explanations of DC spectra are also discussed.