  Quantum dynamical semigroups provide a general framework for studying the
evolution of open systems. Neutrino propagation both in vacuum and in matter
can be analyzed using these techniques: they allow a consistent treatment of
non-standard, dissipative effects that can alter the pattern of neutrino
oscillations. In particular, initially massless neutrinos can give rise to a
nonvanishing flavour transition probability, involving in addition the Majorana
CP-violating mixing phase.
