Abstract.Even if neutrino masses are unknown, we know neutrinos are much lighter than the other fermions we know, and we do not have a good explanation for it. In the Standard Model of elementary particles neutrinos are exactly massless, although this is not insured by any basic principle. Non-zero neutrino masses arise in many extensions of the Standard Model. Massive neutrinos and their associated properties, such as the Dirac or Majorana character of neutrinos, their mixings, lifetimes and magnetic or electric moments, may have very important consequences in astrophysics, cosmology and particle physics. Here we explore these consequences and the constraints they already impose on neutrino properties, as well as the large body of experimental and observational efforts currently devoted to elucidate the mystery of neutrino masses.Several hints for non-zero masses in solar and atmospheric neutrinos, that will be confirmed or rejected in the near future, make this field of research particularly exciting at present.