  Neutrinoless double-beta decay is a forbidden, lepton-number-violating
nuclear transition whose observation would have fundamental implications for
neutrino physics, theories beyond the Standard Model and cosmology. In this
review, we summarize the theoretical progress to understand this process, the
expectations and implications under various particle physics models, as well as
the nuclear physics challenges that affect the precise predictions of the decay
half-life. We will also provide a synopsis of the current and future
large-scale experiments that aim at discovering this process in physically
well-motivated half-life ranges.
