  FeSe is a fascinating superconducting material at the frontier of research in
condensed matter physics. Here we provide an overview on the current
understanding of the electronic structure of FeSe, focusing in particular on
its low energy electronic structure as determined from angular resolved
photoemission spectroscopy, quantum oscillations and magnetotransport
measurements of single crystal samples. We discuss the unique place of FeSe
amongst iron-based superconductors, being a multi-band system exhibiting strong
orbitally-dependent electronic correlations and unusually small Fermi surfaces,
prone to different electronic instabilities. We pay particular attention to the
evolution of the electronic structure which accompanies the
tetragonal-orthorhombic structural distortion of the lattice around 90 K, which
stabilizes a unique nematic electronic state. Finally, we discuss how the
multi-band multi-orbital nematic electronic structure has an impact on the
understanding of the superconductivity, and show that the tunability of the
nematic state with chemical and physical pressure will help to disentangle the
role of different competing interactions relevant for enhancing
superconductivity.
