Origin of the heaviest elements: The rapid neutron-capture process

The rapid neutron-capture process plays a major role in the production of heavy elements from Fe to U. This review surveys the history and current understanding of $r$-process nucleosynthesis, covering new data from atomic and nuclear physics, astrophysical modeling of $r$-process sites, astronomical observations of stellar abundances, and galactic chemical evolution. It includes a timely and thorough discussion of plausible $r$-process astrophysical environments, following the first multimessenger observation of a binary neutron-star merger.