Big Bang Nucleosynthesis

In the first thousand seconds of its evolution the Universe was a primordial nuclear reactor synthesizing the nuclides D, ${}^3$He, ${}^4$He and ${}^7$Li. These messengers from the Big Bang provide a unique window on the early, hot, dense Universe, offering the opportunity for tests of the standard models of particle physics and of cosmology. A new, statistically coherent comparison of the primordial abundances of these nuclides inferred from increasingly accurate observational data with those predicted by the standard model hints at a possible crisis. The case for this emerging crisis is presented and several paths towards ameliorating it are explored.