In this talk, we extend the Holmström and Milgrom problem by adding uncertainty about the volatility of the output for both the Agent and the Principal. We study more precisely the impact of the "Nature" playing against the Agent and the Principal by choosing the worst possible volatility of the output. We solve the first–best and the second-best problems associated with this framework and we show that optimal contracts are in a class of contracts similar to Cvitanic, Possamaï and Touzi, linear with respect to the output and its quadratic variation. We compare our results with the classical problem. Speaker(s): Thibaut Mastrolia (Ecole Polytechnique)