The history and status of neutrino oscillation experiments are summarized.
If neutrinos are massive, there will be neutrino oscillation.
The main parts of this dissertation include three topics on different aspects of neutrino oscillation and neutrino masses.
These results imply that there are neutrino oscillations, this means that neutrino has mass.
The second topic is about neutrino oscillation theory in quantum field theory(QFT).