  The properties of starbursts in ultraluminous infrared galaxies are
discussed, with particular emphasis on the fueling, the amount of extinction
and the intrinsic properties of the nuclear starbursts. It is shown by the
example of NGC6240 that the H_2 vibrational lines can be used to measure the
rate of gas inflow into the potential well, which is sufficient to fuel a
nuclear starburst of the intensity required to account for the far-infrared
emission. It is shown that in Arp220 the faintness of all tracers of ionized
gas can be accounted for by Lyman continuum absorption by dust within the
ionized regions, combined with significant (but not extreme) extinction; there
is no reason to invoke the presence of extreme extinction, an old starburst, or
an additional non-stellar power source in Arp220.
