The primordial helium-4 abundance, measured from the spectra of metal-poor galaxies and inferred from Big Bang Nucleosynthesis calculations, exhibits an observed value that is slightly but consistently higher than some standard Lambda-CDM predictions expect, particularly when combined with other light element constraints. The tension arises from uncertainties in nuclear reaction rates, systematic errors in abundance measurements from stellar spectra, and the possibility that the early-universe conditions (neutron-to-proton ratio, expansion rate, or baryon density) differed from standard assumptions (Cyburt et al. 2016; Aver et al. 2015).