Abstract. We obtain sharp rates of convergence in the usual sup-norm for the nth iterates D n f and C n f of continuous and discrete Cesàro operators, respectively. In both cases the best possible rate of convergence is n −1/2 , and such a rate is attained under appropriate integrability conditions on f . Otherwise, the rates of convergence could be extremely poor, depending on the behavior of f near the boundary. We introduce probabilistic representations of D n f and C n f involving standardized sums of independent identically distributed random variables and binomial mixtures, respectively, which allow us to use the classical Berry-Esseen theorem.