The design principle of internal model control (IMC) focuses on developing a feedforward controller which also acts in the feedback loop to attenuate disturbances. This paper uses ideas developed in the theory of flat systems for the design of feedforward controllers and adapts them to IMC control. By extending this idea, a method for output feedback control of nonlinear flat systems is presented. It shows the interesting result that the classical design procedure for IMC controllers and the new flatness-based approach end up with identical results for linear systems.