The author reviews the status of our understanding of nucleosynthesis of the light nuclei ($Z<\mathrm{}6\mathrm{}$). The standard view today is that these elements are, for the most part, generated by two different processes: first, thermonuclear reactions in the early universe (big-bang nucleosynthesis or (BBN), and second, galactic cosmic-ray-induced spallation reactions (GCR) in cold interstellar atoms. A third contribution comes from stellar processes. The arguments in favor of this view are presented. Numerous astrophysical and cosmological implications are discussed, such as the baryonic density, the possible existence of baryonic dark matter and of nonbaryonic (exotic) matter, the constraints imposed on new particle physics, the leptonic number of the universe, the increase in cosmic entropy since primordial nucleosynthesis, and the constancy of the "constants" of physics.

DOI:https://doi.org/10.1103/RevModPhys.66.193