  Gaia is the cornerstone mission of the European Space Agency. From late 2013
it will start collecting superb astrometric, photometric and spectroscopic data
for around a billion of stars of our Galaxy. While surveying the whole sky down
to V=20mag Gaia will be detecting transients and anomalous behaviour of
objects, providing near-real-time alerts to the entire astronomical community.
Gaia should detected about 6000 supernovae, 1000 microlensing events and many
other interesting types of transients. Thanks to its on-board low-dispersion
spectrograph the classification of transients will be robust, assuring low
false-alert rate. We describe the operation of the Photometric Science Alerts
system, outline the scientific possibilities and conclude with an invitation to
collaborate in the ground-based follow-up Gaia alerts during the early months
of the mission when the outcome of the alerting pipeline needs to be verified.
