  Muons can serve as probes to precisely determine fundamental parameters of
the Standard Model or search for `new physics'. The high intensity muon beams
at the Paul Scherrer Institut (PSI) allow for precision measurements and
searches for rare or forbidden processes. Both types of experiments challenge
the Standard Model in a way complementary to high energy physics. We give a
short overview of recent results and ongoing experiments at PSI, and of ideas
for the future.
