  In the Standard Model the total lepton number is conserved. Thus,
neutrinoless double-beta decay, in which the total lepton number is violated by
two units, is a probe of physics beyond the Standard Model. In this review we
consider the basic mechanism of neutrinoless double-beta decay induced by light
Majorana neutrino masses. After a brief summary of the present status of our
knowledge of neutrino masses and mixing and an introduction to the seesaw
mechanism for the generation of light Majorana neutrino masses, in this review
we discuss the theory and phenomenology of neutrinoless double-beta decay. We
present the basic elements of the theory of neutrinoless double-beta decay, our
view of the present status of the challenging problem of the calculation of the
nuclear matrix element of the process and a summary of the experimental
results.
