This chapter reframes density as a core force shaping China's urban future. It defines how density is measured, why thresholds vary, and how perceptions diverge from numberical metrics. It tracks the evolution of density theories, contrasting centralization and compact city models. It assesses density's trade-offs, efficient land use and vitality on one side, congestion and infrastructural stress on the other. It argues that densification must be contextual, promoting "moderate density" as a balanced, adaptable strategy for sustainable regeneration in China's high-density cities.