  Many of the conditions that are necessary for starbursts appear to be
important in the triggering of QSOs. However, it is still debatable whether
starbursts are ubiquitously present in galaxies harboring QSOs. In this paper
we review our current knowledge from observations of the role of starbursts in
different types of QSOs. Post-starburst stellar populations are potentially
present in the majority of QSO hosts. QSOs with far-infrared colors similar to
those of ultraluminous infrared galaxies invariably reside in merging galaxies
that have interaction-induced starbursts of a few hundred Myr or less. Similar,
but dramatically more luminous post-starburst populations are found in the
recently discovered class of QSOs known as post-starburst QSOs, or Q+A's. Both
of these classes, however, comprise only a small fraction (10-15%) of the total
QSO population. The so-called "red" QSOs generally suffer from strong
extinction at optical wavelengths, making them ideal candidates for the study
of hosts. Their stellar populations typically show a post-starburst component
as well, though with a larger range of ages. Finally, optical "classical" QSO
hosts show traces of major star formation episodes (typically involving >10% of
the mass of the stellar component) in the more distant past (1-2 Gyr). These
starbursts appear to be linked to past merger events. It remains to be
determined whether these mergers were also responsible for triggering the QSO
activity that we observe today.
