  Neutrinos in the Standard Model of particle physics are massless, neutral
fermions that seemingly do little more than conserve 4-momentum, angular
momentum, lepton number, and lepton flavour in weak interactions. In the last
decade conclusive evidence has demonstrated that the Standard Model's
description of neutrinos does not match reality. We now know that neutrinos
undergo flavour oscillations, violating lepton flavour conservation and
implying that neutrinos have non-zero mass. A rich oscillation phenomenology
then becomes possible, including matter-enhanced oscillation and possibly CP
violation in the neutrino sector. Extending the Standard Model to include
neutrino masses requires the addition of new fields and mass terms, and
possibly new methods of mass generation. In this review article I will discuss
the evidence that has established the existence of neutrino oscillation, and
then highlight unresolved issues in neutrino physics, such as the nature of
three-generational mixing (including CP-violating effects), the origins of
neutrino mass, the possible existence of light sterile neutrinos, and the
difficult question of measuring the absolute mass scale of neutrinos.
