This paper discusses the role of the dilaton field in string theory and its implications for the coupling of matter to gravity. The authors propose that non-perturbative string loop effects can naturally reconcile the existence of a massless dilaton with experimental data, leading to a "Least Coupling Principle" where the dilaton decouples from matter during cosmological evolution. This principle suggests that the dilaton's coupling to matter becomes weaker over time, resulting in small but observable deviations from general relativity. The authors also show that the presence of a weakly coupled massless dilaton implies a large spectrum of deviations from Einstein's Equivalence Principle, motivating further experimental tests of this principle. The paper analyzes the cosmological evolution of the dilaton in the context of string theory, showing that the dilaton is attracted to values where it decouples from matter. The authors provide quantitative estimates of the residual coupling strength of the dilaton at the present cosmological epoch and discuss the implications for the observed abundance of light elements, such as helium. The results suggest that a dilatonic universe could naturally accommodate a universe with a total baryon mass density equal to the closure density, Ω_b = 1, if the dilaton's value at freeze-out differs slightly from its minimum. The paper concludes that the cosmological evolution of the dilaton is an extremely efficient way of pinning down its value, leading to a "Principle of Least Coupling" where the universe is attracted to dilaton values that minimize the strength of interactions.This paper discusses the role of the dilaton field in string theory and its implications for the coupling of matter to gravity. The authors propose that non-perturbative string loop effects can naturally reconcile the existence of a massless dilaton with experimental data, leading to a "Least Coupling Principle" where the dilaton decouples from matter during cosmological evolution. This principle suggests that the dilaton's coupling to matter becomes weaker over time, resulting in small but observable deviations from general relativity. The authors also show that the presence of a weakly coupled massless dilaton implies a large spectrum of deviations from Einstein's Equivalence Principle, motivating further experimental tests of this principle. The paper analyzes the cosmological evolution of the dilaton in the context of string theory, showing that the dilaton is attracted to values where it decouples from matter. The authors provide quantitative estimates of the residual coupling strength of the dilaton at the present cosmological epoch and discuss the implications for the observed abundance of light elements, such as helium. The results suggest that a dilatonic universe could naturally accommodate a universe with a total baryon mass density equal to the closure density, Ω_b = 1, if the dilaton's value at freeze-out differs slightly from its minimum. The paper concludes that the cosmological evolution of the dilaton is an extremely efficient way of pinning down its value, leading to a "Principle of Least Coupling" where the universe is attracted to dilaton values that minimize the strength of interactions.