The controlled anionic ring-opening polymerization of lactones and siloxanes is carried out in continuous-flow reactors. Anionic ring-opening polymerization of cyclic esters with strong, soluble bases such as potassium tert-butoxide (KOtBu) is typically performed as batch reactions, leading to broad dispersity and poor control over the molecular weight (Mn) mainly because of transesterification reactions. Although reactions with strong bases give rise to “uncontrolled” polymerizations, a variety of transition metal-based and organocatalysts have been developed with the goal of reliably controlling the dispersity and Mn. Herein, we show that the rapid mixing and short residence times accessible with a continuous-flow apparatus can enable the controlled polymerization of low-reactivity monomers such as δ-valerolactone and εcaprolactone on millisecond time scales using a base [such as KOtBu or potassium bis(trimethylsilyl)amide (KHMDS)] in conjunction with a primary alcohol. These reactions exhibit characteristics capable of producing narrow dispersity with predictable molecular weights and can rapidly generate well-defined block copolymers with residence times below 0.1 s.