  We discuss unitarity tests of the neutrino mixing (PMNS) matrix. We show that
the combination of solar neutrino experiments, medium-baseline and
short-baseline reactor antineutrino experiments make it possible to perform the
first direct unitarity test of the PMNS matrix. In particular, the measurements
of Daya Bay and JUNO (a next generation medium-baseline reactor experiment)
will lay the foundation of a precise unitarity test of $|U_{e1}|^2 + |U_{e2}|^2
+ |U_{e3}|^2 = 1 $. Furthermore, the precision measurement of
$\sin^22\theta_{13}$ in both the $\bar{\nu}_e$ disappearance and the $\nu_e$
appearance (from a $\nu_{\mu}$ beam) channels will provide an indirect
unitarity test of the PMNS matrix. Together with the search for
appearance/disappearance at very short distances, these tests could provide
important information about the possible new physics beyond the three-neutrino
model.
