  Since the discovery of neutrino oscillations, the experimental progress in
the last two decades has been very fast, with the precision measurements of the
neutrino squared-mass differences and of the mixing angles, including the last
unknown mixing angle $\theta_{13}$.
  Today a very large set of oscillation results obtained with a variety of
experimental configurations and techniques can be interpreted in the framework
of three active massive neutrinos, whose mass and flavour eigenstates are
related by a 3 $\times$ 3 unitary mixing matrix, the
Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix, parameterized by three mixing
angles $\theta_{12}$, $\theta_{23}$, $\theta_{13}$ and a CP-violating phase
$\delta_{CP}$. The additional parameters governing neutrino oscillations are
the squared-mass differences $\Delta m^2_{ji}=m^2_j-m^2_i$, where $m_i$ is the
mass of the $i$th neutrino mass eigenstate. This review covers the rise of the
PMNS three-neutrino mixing paradigm and the current status of the experimental
determination of its parameters.
  The next years will continue to see a rich program of experimental endeavour
coming to fruition and addressing the three missing pieces of the puzzle,
namely the determination of the octant and precise value of the mixing angle
$\theta_{23}$, the unveiling of the neutrino mass ordering (whether $m_1 < m_2
< m_3$ or $m_3 < m_1 < m_2$) and the measurement of the CP-violating phase
\dcp.
