The moduli space of D-branes has a microscopic closed string description in terms of a continuous family of boundary states. Open string field theory provides a new microscopic description in terms of non-perturbative configurations of open strings which solve the open string equation of motion. In this talk, after a general introduction to the subject, I will explain how the SFT parameters are non-trivially related to the boundary state parameters. Using a recently discovered analytic solution I show that, for large values of the moduli, the motion in moduli space is not driven by the open string marginal field but it is a collective effect of all the massive modes. By a numerical computation to high level, this behavior is shown to be true also in Feynman-Siegel gauge.
