  The formation of a star is a dynamic process fed by the gravitational
collapse of a molecular cloud core. Theoretical models and observations suggest
that the majority of this infalling material settles into a protoplanetary disk
before reaching the (proto)star and therefore that disk accretion processes are
responsible for the rate at which the (proto)star grows. There is no
fundamental reason why infall and disk accretion need to be instantaneously
identical. Indeed, even within the disk it might be anticipated that there are
regions of strong and weak accretion. Together these facts suggest that
(proto)stellar mass assembly should be both secular and stochastic and that the
underlying physical processes leading to these time-variable accretion rates
should manifest in observable time-dependent accretion luminosity variations.
