Theoretical models on the structure of the neutrino mixing matrix

We briefly review symmetry approaches to the flavour problem in the lepton sector, focusing on the structure of the neutrino mixing matrix. We first discuss traditional (linearly realised) non-Abelian discrete flavour symmetries and then modular flavour symmetries. The importance of future neutrino oscillation experiments for testing and narrowing down the broad classes of flavour models based on these symmetries is emphasised.