  The Mu3e experiment at the Paul Scherrer Institut will search for the
lepton-number-violating decay \mueee, extending the sensitivity by four orders
of magnitude compared to existing limits. This probe of new physics is
complementary to the existing collider, dark matter and neutrino particle
physics programmes, and part of a global programme investigating the charged
lepton flavour sector. As well as the main \mueee search, Mu3e will also extend
the sensitivity to low-mass dark photons, and additional flavour-violating
decays involving long-lived or stable particles.
