As the hot, dense, early Universe rushed to expand and cool, it briefly passed through the epoch of big bang nucleosynthesis (BBN), leaving behind as relics the first complex nuclei: deuterium, helium-3, helium-4, and lithium-7. The abundances of these relic nuclides were determined by the competition between the relative densities of nucleons (baryons) and photons and, by the universal expansion rate. In particular, while deuterium is an excellent baryometer, 4He provides an accurate chronometer. Nearly 400 thousand years later, when the cosmic background radiation (CBR) had cooled sufficiently to allow neutral atoms to form, releasing the CBR from the embrace of the ionized plasma of protons and electrons, the spectrum of temperature fluctuations imprinted on the CBR encoded the baryon and radiation densities, along with the universal expansion rate at that epoch. As a result, the relic abundances of the light nuclides and the CBR temperature fluctuation spectrum provide invaluable windows on the early evolution of the Universe along with sensitive probes of its particle content.
The fruitful interplay between theory and data has been key to the enormous progress in cosmology in recent times. As new, more precise data became available, models have had to be refined or rejected. It is anticipated this this process will - indeed, should - continue. Therefore, this review of the baryon content of the Universe as revealed by BBN and the CBR is but a signpost on the road to a more complete understanding of the history and evolution of the Universe. By highlighting the current successes of the present "standard" model along with some of the challenges to it, I hope to identify those areas of theoretical and observational work which will contribute to continuing progress in our endeavor to understand the Universe, its past, present, and future.