Finding the organising principle of the flavour sector is one of the big challenges in particle physics:
a) why are there three generations of fermions?
b) why is the up quark about 100,000 times lighter than the top quark, although they have the same gauge quantum numbers?
c) why do the three generations of quarks hardly mix, whereas the three lepton generations have large mixing?
d) could there be more than three generations?
e) how many neutrinos are there?
f) could there be also more generations of Higgs particles?
In this talk, different theoretical ideas and possible experimental tests will be discussed.