  This paper describes eruptive phenomena in pre-main sequence stars. The
eruptions of FU Orionis stars have much in common with outbursts in other
accreting systems, such as dwarf novae and some symbiotic stars. These common
features are best understood as increases in the rate material flows through an
accretion disk. The spectroscopic properties, decay of the light curves, and
outflow phenomena suggest that these outbursts arise from thermal instabilities
in the disk. Available data and estimates for recurrence times indicate that
young stars can accrete much, perhaps all, of their mass in FU Ori accretion
events. Future observations can test this notion and place better constraints
on the importance of eruptive events in the early life of a low mass star.
