We examine the design of optimal rating systems in the presence of moral hazard. First, an intermediary commits to a rating scheme. Then, a decision-maker chooses an action that generates value for the buyer. The intermediary then observes a noisy signal of the decision-maker's choice and sends the buyer a signal consistent with the rating scheme. Here we fully characterize the set of allocations that can arise in equilibrium under any arbitrary rating system. We use this characterization to study various design aspects of optimal rating systems. Specifically, we study the properties of optimal ratings when the decision-maker's effort is productive and when the decision-maker can manipulate the intermediary's signal with a noise. With manipulation, rating uncertainty is a fairly robust feature of optimal rating systems.