Materials like Bi-Sb alloys, Bi2Se3 or Bi2Te3 have been known and studied for decades, mostly for their interesting thermoelectrical properties. They are insulators with relatively small band gaps, which are opened by the spin-orbit interaction. Recently, it was recognized that the topology of the electronic states separated by this gap is non-trivial. Jürg Osterwalder gives an introduction to the new aspects of these materials.