We review the Cosmology and Physics underlying Primordial Nucleosynthesis and
survey current observational data in order to compare the predictions of Big
Bang Nucleosynthesis with the inferred primordial abundances. From this
comparison we report on the status of the consistency of the standard hot big
bang model, we constrain the universal density of baryons (nucleons), and we
set limits to the numbers and/or effective interactions of hypothetical new
"light" particles (equivalent massless neutrinos).Comment: 25 pages, latex, 4 ps figures, to be published in a special memorial
volume of Physics Reports in honor of David Schram