The history of research associated with the fundamental problem of the nature of massive neutrinos, which can be Dirac particles or Majorana fermions, and the related processes of neutrinoless double beta decay, (A,Z)→(A,Z+2)+e−+e−, and two neutrino double beta decay, (A,Z)→(A,Z+2)+e−+e−+νe ̄+νe ̄, is reviewed.
