This research monograph provides a comprehensive demographic analysis of Korea’s unprecedented fertility decline by integrating fertility elasticity analysis, decomposition analysis, and expert survey evidence. Focusing on the sharp downturn since 2015, it identifies a structural shift in the drivers of fertility change, underscoring the growing importance of marriage and first births. By offering robust empirical insights into evolving demographic mechanisms, the study contributes to evidence-based debates on reorienting Korea’s fertility and family policy framework in the era of ultra-low fertility.