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