  We study the possible structure of the neutrino mass matrix taking into
consideration the solar and atmospheric neutrinos and the neutrinoless double
beta decay. We emphasize on mass matrices with vanishing elements. There are
only a very few possibilities remaining at present. We concentrate on three
generation scenarios and find that with three parameters there are few
possibilities with and without any vanishing elements. For completeness we also
present a five parameter four neutrino (with one sterile neutrino) mass matrix
which can explain all these experiments and the LSND result.
