  Neutrinoless double beta decay has been the subject of intensive theoretical
work as it represents the only practical approach to discovering whether
neutrinos are Majorana particles or not, and whether lepton number is a
conserved quantum number. Available calculations of matrix elements and
phase-space factors are reviewed from the perspective of a future large-scale
experimental search for neutrinoless double beta decay. Somewhat unexpectedly,
a uniform inverse correlation between phase space and the square of the nuclear
matrix element emerges. As a consequence, no isotope is either favored or
disfavored; all have qualitatively the same decay rate per unit mass for any
given value of the Majorana mass.
