  We present a detailed quantitative discussion of the measurement of the
leptonic mixing angle $\sin^2 2 \theta_{13}$ with a future reactor neutrino
oscillation experiment consisting of a near and far detector. We perform a
thorough analysis of the impact of various systematical errors and compare the
resulting physics potential to the one of planned first-generation superbeam
experiments. Furthermore, we investigate the complementarity of both types of
experiments. We find that, under realistic assumptions, a determination of
$\sin^2 2 \theta_{13}$ down to $10^{-2}$ is possible with reactor experiments.
They are thus highly competitive to first-generation superbeams and may be able
to test $\sin^2 2 \theta_{13}$ on shorter timescales. In addition, we find that
the combination of a KamLAND-size reactor experiment with one or two superbeams
could substantially improve the ability to access the neutrino mass hierarchy
or the leptonic CP phase.
