Revised: December 2019 Kolotilin gratefully acknowledges financial support from the Australian Research Council Discovery Early Career Research Award DE160100964 and from MIT Sloan's Program on Innovation in Markets and Organizations. Zapechelnyuk gratefully acknowledges financial support from the Economic and Social Research Council Grant ES/N01829X/1.A sender designs a signal about the state of the world to persuade a receiver. Under standard assumptions, an optimal signal reveals the states below a cutoff and pools the states above the cutoff. This result holds in continuous and discrete environments. The optimal signal is less informative if the sender is more biased and if the receiver is easier to persuade. We apply our results to the...