  Exploration of the time domain - variable and transient objects and phenomena
- is rapidly becoming a vibrant research frontier, touching on essentially
every field of astronomy and astrophysics, from the Solar system to cosmology.
Time domain astronomy is being enabled by the advent of the new generation of
synoptic sky surveys that cover large areas on the sky repeatedly, and
generating massive data streams. Their scientific exploration poses many
challenges, driven mainly by the need for a real-time discovery,
classification, and follow-up of the interesting events. Here we describe the
Catalina Real-Time Transient Survey (CRTS), that discovers and publishes
transient events at optical wavelengths in real time, thus benefiting the
entire community. We describe some of the scientific results to date, and then
focus on the challenges of the automated classification and prioritization of
transient events. CRTS represents a scientific and a technological testbed and
precursor for the larger surveys in the future, including the Large Synoptic
Survey Telescope (LSST) and the Square Kilometer Array (SKA).
