This paper presents a mechanism for generating the large-scale curvature perturbation in the universe through the late decay of a massive scalar field, the curvaton. The curvaton is light during inflation and acquires a perturbation with a nearly scale-invariant spectrum. Initially, this corresponds to an isocurvature density perturbation, which generates the curvature perturbation after inflation when the curvaton's density becomes significant. The isocurvature perturbation disappears if the curvaton completely decays into thermalized radiation. Any residual isocurvature perturbation is 100% correlated with the curvature. The same mechanism can generate curvature perturbations in pre-big bang/ekpyrotic models if the curvaton has a suitable non-canonical kinetic term. The curvaton field generates the curvature perturbation in two stages: first, its quantum fluctuation during inflation is converted into a classical perturbation with a flat spectrum at horizon exit. Then, after inflation, the curvaton field's perturbation is converted into a curvature perturbation. This mechanism does not require assumptions about inflation beyond the Hubble parameter being nearly constant. The curvaton's properties and the cosmology after inflation determine the curvature perturbation. The paper shows that the quantum fluctuation of the curvaton during inflation is converted into a curvature perturbation after decay according to the formula ζ ∼ rδ, where δ is the isocurvature fractional density perturbation in the curvaton before decay and r is the fraction of the final radiation produced by the decay. The mechanism is described as the conversion of an isocurvature perturbation into a curvature perturbation. It was discovered over a decade ago by Mollerach, who corrected the misconception that no conversion would occur. The paper discusses the curvaton field, its potential, and the conditions under which it generates curvature perturbations. It also explores the curvaton's role in generating isocurvature density perturbations and its implications for cosmological observations. The paper concludes that the curvature perturbation can be generated by the curvaton without any accompanying isocurvature perturbation at late times, and that the curvaton's decay before neutrino decoupling is essential for this. The paper also discusses the curvaton as a flat direction and a pseudo-goldstone boson, and its implications for inflationary models. It concludes that the curvature perturbation in the universe need not be generated by the quantum fluctuation of a slowly-rolling inflaton field, but can be generated by the quantum fluctuation of a field unrelated to the inflation model, the curvaton.This paper presents a mechanism for generating the large-scale curvature perturbation in the universe through the late decay of a massive scalar field, the curvaton. The curvaton is light during inflation and acquires a perturbation with a nearly scale-invariant spectrum. Initially, this corresponds to an isocurvature density perturbation, which generates the curvature perturbation after inflation when the curvaton's density becomes significant. The isocurvature perturbation disappears if the curvaton completely decays into thermalized radiation. Any residual isocurvature perturbation is 100% correlated with the curvature. The same mechanism can generate curvature perturbations in pre-big bang/ekpyrotic models if the curvaton has a suitable non-canonical kinetic term. The curvaton field generates the curvature perturbation in two stages: first, its quantum fluctuation during inflation is converted into a classical perturbation with a flat spectrum at horizon exit. Then, after inflation, the curvaton field's perturbation is converted into a curvature perturbation. This mechanism does not require assumptions about inflation beyond the Hubble parameter being nearly constant. The curvaton's properties and the cosmology after inflation determine the curvature perturbation. The paper shows that the quantum fluctuation of the curvaton during inflation is converted into a curvature perturbation after decay according to the formula ζ ∼ rδ, where δ is the isocurvature fractional density perturbation in the curvaton before decay and r is the fraction of the final radiation produced by the decay. The mechanism is described as the conversion of an isocurvature perturbation into a curvature perturbation. It was discovered over a decade ago by Mollerach, who corrected the misconception that no conversion would occur. The paper discusses the curvaton field, its potential, and the conditions under which it generates curvature perturbations. It also explores the curvaton's role in generating isocurvature density perturbations and its implications for cosmological observations. The paper concludes that the curvature perturbation can be generated by the curvaton without any accompanying isocurvature perturbation at late times, and that the curvaton's decay before neutrino decoupling is essential for this. The paper also discusses the curvaton as a flat direction and a pseudo-goldstone boson, and its implications for inflationary models. It concludes that the curvature perturbation in the universe need not be generated by the quantum fluctuation of a slowly-rolling inflaton field, but can be generated by the quantum fluctuation of a field unrelated to the inflation model, the curvaton.