  The leptonic mixing angle theta_13 is currently a high-priority topic in the
field of neutrino physics, with five experiments under way, searching for
neutrino oscillations induced by this angle. We review the phenomenology of
theta_13 and discuss the information from present global oscillation data. A
description of the upcoming reactor and accelerator experiments searching for a
non-zero value of theta_13 is given, and we evaluate the sensitivity reach
within the next few years.
