Exploration of the Time Domain

Synoptic sky surveys are becoming the dominant data sources in astronomy, and
thus offer both new scientific opportunities and methodological challenges.
This effectively opens a new research frontier, that touches essentially every
field of astronomy, from the Solar system to cosmology, and from stellar
evolution to extreme relativistic astrophysics.  Opening of the time domain to
a systematic exploration will strengthen our understanding of a number of
interesting known phenomena, and may lead to the discoveries of as yet unknown
ones.  We describe some lessons learned over the past decade, and take a broad
view of this field.