Episodic Accretion in Young Stars
  In the last twenty years, the topic of episodic accretion has gained
significant interest in the star formation community. It is now viewed as a
common, though still poorly understood, phenomenon in low-mass star formation.
The FU Orionis objects (FUors) are long-studied examples of this phenomenon.
FUors are believed to undergo accretion outbursts during which the accretion
rate rapidly increases from typically $10^{-7}$ to a few $10^{-4}$ $M_\odot$
yr$^{-1}$, and remains elevated over several decades or more. EXors, a loosely
defined class of pre-main sequence stars, exhibit shorter and repetitive
outbursts, associated with lower accretion rates. The relationship between the
two classes, and their connection to the standard pre-main sequence
evolutionary sequence, is an open question: do they represent two distinct
classes, are they triggered by the same physical mechanism, and do they occur
in the same evolutionary phases? Over the past couple of decades, many
theoretical and numerical models have been developed to explain the origin of
FUor and EXor outbursts. In parallel, such accretion bursts have been detected
at an increasing rate, and as observing techniques improve each individual
outburst is studied in increasing detail. We summarize key observations of
pre-main sequence star outbursts, and review the latest thinking on outburst
triggering mechanisms, the propagation of outbursts from star/disk to disk/jet
systems, the relation between classical EXors and FUors, and newly discovered
outbursting sources -- all of which shed new light on episodic accretion. We
finally highlight some of the most promising directions for this field in the
near- and long-term.
