Big bang nucleosynthesis (BBN), an epoch of primordial nuclear transformations in the expanding Universe, has become a cornerstone of modern cosmology. I will first review the BBN framework and then discuss how it can be used as a toolbox for testing physics beyond the Standard Model. In particular, scenarios in which a massive particle
species X decays during or after BBN have a long-standing tradition. There, I will point out two new results: First, if X is a GeV-scale relic, then a generic solution to the cosmological lithium problem is possible. Such light sectors (containing X) have received a fair amount of interest in the recent past. Second, if X is an electroweak-scale relic, then its decay can give rise to the formation of primordial beryllium. Such models then become testable via atmospheric measurements of 9Be in extremely metal-poor halo stars.