Observations of galaxies and star-forming regions suggest an apparent “metallicity floor,” a lower bound on gas and stellar metallicities (often at a few thousandths to a few percent of solar) that is rarely, if ever, undercut, even in very low-mass systems and at high redshift (Kunth & Östlin 2000; Wise et al. 2012). In ΛCDM this floor is usually attributed to early, widespread enrichment by the first stars and galaxies, but matching its value and universality requires assumptions about Population III star formation, metal mixing, and feedback that are still highly uncertain, and simulations must often impose a metallicity floor by hand to avoid forming unrealistically metal-free gas and stars (Wise et al. 2012; Jaacks et al. 2018).