Non-zero neutrino masses indicate physics beyond the Standard Model (BSM). Neutrinoless double beta decay (0νββ) is a hypothetical process which, if discovered, would provide a neutrino mass generation mechanism, establish neutrinos being their own antiparticles, and prove total lepton number violation. 0νββ is expected to be extremely rare, resulting in experimental requirement of unprecedentedly low backgrounds. Recent 0νββ experiments based on high purity germanium detectors have achieved great successes including the best energy resolution and the lowest background in the field. The next generation Ge-based 0νββ project, LEGEND, has started the commissioning of its initial phase.
This talk will review the physics of 0νββ. Final results from the current generation Ge-based 0νββ experiments will be summarized, and the status of LEGEND will be discussed. The combination of ultra-low background and superb energy performance enables a broad and rich physics program beyond 0νββ for Ge-based experiments, including both standard model nuclear physics and BSM physics such as axions, dark matter, and more. Physics opportunities beyond 0νββ will be outlined and new results will be presented.