Slowly Changing Dimensions (SCDs) are how data systems manage attributes that evolve without constantly rewriting history. They determine whether you keep only the latest value, preserve full historical versions, or maintain a limited snapshot of changes. The classic SCD types (0–3, plus hybrids) define different behaviours… from never updating values, to overwriting them, to keeping every version with timestamps. The real purpose of SCDs is to make an explicit choice about how truth should behave in your analytics: what should remain fixed, what should update, and what historical context matters. Modern data platforms make tracking changes easy, but they don’t make the design decisions for you. SCDs are ultimately the backbone of reliable, temporal, reality-preserving analytics.