Big Bang nucleosynthesis occurs within the first few minutes of cosmic history and predicts the primordial abundances of D, $^3$He, $^4$He, and $^7$Li. Its success is a major validation of the hot Big Bang model and a sensitive probe of the baryon density and relativistic degrees of freedom.

Before weak freeze-out, neutrons and protons interconvert through weak interactions. Once the weak rate drops below $H$, the neutron-to-proton ratio freezes approximately as $$\frac{n}{p} \approx e^{-\Delta m/T_f},$$ where $\Delta m$ is the neutron-proton mass difference and $T_f$ is the freeze-out temperature.

Accurate BBN requires a coupled network of nuclear reactions, radiative corrections, neutrino heating, and updated cross sections. Theoretical uncertainties are propagated through Monte Carlo or covariance-based methods to compare with observational abundance estimates.

BBN constrains extra radiation, decaying relics, lepton asymmetry, variations in fundamental constants, and beyond-standard-model particle content. It therefore provides an important complement to CMB constraints on $N_{\rm eff}$ and the expansion history.