Despite their importance in activated processes, transition-event durations -which are much shorter than first passage times -have not received a complete theoretical treatment. We therefore study the distribution ρ b (t) of durations of transition events over a barrier in a one-dimensional system undergoing over-damped Langevin dynamics. We show that ρ b (t) is determined by a Fokker-Planck equation with absorbing boundary conditions, and obtain a number of results, including: (i) the analytic form of the asymptotic short-time behavior (t → 0), which is universal and independent of the potential function; (ii) the first non-universal correction to the short-time behavior; (iii) following Gardiner [1], a recursive formulation for calculating, exactly, all moments of ρ b based solely on the potential function -along with approximations for the distribution based on a small number of moments; and (iv) a high-barrier approximation to the long-time (t → ∞) behavior of ρ b (t). We also find that the mean event duration does not depend simply on the barrier-top frequency (curvature), but is sensitive to details of the potential. All of the analytic results are confirmed by transition-path-sampling simulations, implemented in a novel way.