  Behind the observed pattern of lepton flavor mixing is a partial or
approximate mu-tau flavor symmetry --- a milestone on our road to the true
origin of neutrino masses and flavor structures. In this review article we
first describe the features of mu-tau permutation and reflection symmetries,
and then explore their various consequences on model building and neutrino
phenomenology. We pay particular attention to soft mu-tau symmetry breaking,
which is crucial for our deeper understanding of the fine effects of flavor
mixing and CP violation.
