M. Audard (University of Geneva, Switzerland),
P. Abraham (Konkoly Observatory, Hungary),
M. Dunham (Yale University, United States),
J. Green (University of Texas, Austin, United States),
N. Grosso (Observatoire Astronomique de Strasbourg, France),
K. Hamaguchi (NASA GSFC, United States),
J. Kastner (Rochester Institute of Technology, United States),
A. Kospal (ESA, Netherlands),
G. Lodato (University of Milan, Italy),
M. Romanova (Cornell University, United States),
S. Skinner (University of Colorado, United States),
E. Vorobyov (University of Vienna, Austria),
Z. Zhu (Princeton University, United States)

Since the last major review in Protostars and Planets IV, 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 (FUor) 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} Msun/yr to 10^(-4) Msun/yr, and remains elevated over timescales of several decades or more. EXors, a loosely defined class of pre-main sequence stars, exhibit similar but shorter and repetitive outbursts, associated with lower accretion rates. The relationship between the two classes, and their relationship to the standard pre-main sequence evolutionary sequence, is an open question: do they form 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 increasing rate, and each individual outburst is more carefully scrutinized and monitored across the electromagnetic spectrum. These rapid advances have led our team of observers and theorists to review our contemporary understanding of episodic accretion from the theoretical, numerical, and observational points of view, and to highlight the most promising directions for this field of star formation in the near- and long-term. We summarize key observations of pre-main sequence star outbursts across the electromagnetic spectrum, and review the latest thinking on outburst triggering mechanisms, the propagation of outbursts from star/disk to disk/jet systems, the relationships between classical EXors and FUors, and newly discovered outbursting sources – all of which shed new light on episodic accretion.

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