On vast geological timescales, significant sea-level (SL) variations are driven by two fundamental forces: the reshaping of ocean basins, which alters the very capacity of the globe’s watery cradle, and the shifting volume of free water within these basins, dictated by the waxing and waning of ice sheets on land and other hydrological processes. In this chapter, we turn our focus to the grand-scale forces that shape SL trajectories over spans of thousands to millions of years—the drivers that etch their legacy into Earth’s sedimentary layers and isotopic archives, preserving a record of the planet’s dynamic climates and oceans.