  Gaia is a European Space Agency (ESA) astrometry space mission, and a
successor to the ESA Hipparcos mission. Gaia's main goal is to collect
high-precision astrometric data (i.e. positions, parallaxes, and proper
motions) for the brightest 1 billion objects in the sky. These data,
complemented with multi-band, multi-epoch photometric and spectroscopic data
collected from the same observing platform, will allow astronomers to
reconstruct the formation history, structure, and evolution of the Galaxy.
  Gaia will observe the whole sky for 5 years, providing a unique opportunity
for the discovery of large numbers of transient and anomalous events, e.g.
supernovae, novae and microlensing events, GRB afterglows, fallback supernovae,
and other theoretical or unexpected phenomena. The Photometric Science Alerts
team has been tasked with the early detection, classification and prompt
release of anomalous sources in the Gaia data stream. In this paper, we discuss
the challenges we face in preparing to use Gaia to search for transient
phenomena at optical wavelengths.
