Neutrinos are some of the most difficult to observe elementary particles. They interact only weakly. Most of them escape without detection, only a tiny fraction of them can be seen. There are three kinds of neutrinos: electron neutrino, muon neutrino and tau neutrino. Neutrinos can oscillate i.e. they can change their identity in flight and transform from one kind of neutrino into another kind of neutrino. Sources of observed neutrinos include: nuclear reactors, accelerator beams, radioactive sources, secondary particles created by interaction of cosmic rays in the atmosphere, Sun, supernovae. Other possible sources are: dark matter, gamma ray bursts, active galactic nuclei, relic neutrinos, Earth interior.