The silicon isotope composition of biogenic silica (δ30SiBSi) in the ocean is a function of the δ30Si of the available dissolved Si (DSi; H2SiO4), the degree of utilisation of the available DSi, and, for some organisms, the concentration of DSi. This makes δ30SiBSi in sediment archives a promising proxy for past DSi concentrations and utilisation. At steady-state, mean δ30SiBSi must equal a weighted average of the inputs, the majority of which are of continental origin. Variation in the functioning of the continental Si cycle on timescales similar to the residence time of DSi in the ocean (~ 10 ka) may therefore contribute to downcore variability in δ30SiBSi on millennial or longer timescales. The direction and magnitude of change in published δ30SiBSi records over the last few glacial cycles are consistent among ocean basins and between groups of silicifiers. They document glacial values that are typically 0.5 to 1.0‰ lower than interglacial values and together hint at coherent and predictable glacial–interglacial variability in whole-ocean δ30Si driven by a change in mean δ30Si of the inputs. In this contribution, we review the modern inputs of DSi to the ocean and the controls on their isotopic composition, and assess the evidence for their variability on millennial-plus timescales.