Possible sources of cosmic neutrinos and features of their production will be outlined in view of recent data on cosmic rays and gamma radiation. Physics of propagation of these neutrinos will be considered which includes oscillations in vacuum, lost of coherence, flavor conversion inside sources. Various non-standard effects will be discussed. Information about the production and propagation is encoded in the neutrino flavor ratios as functions of neutrino energy. Evolution of the flavor ratios and their ``decoding" will be considered.