Straumann discusses modified theories of gravity to explain the acceleration of the universe. This acceleration is currently explained using the hypothesis of Dark Energy. However, DE has not been observed. The method explained in this lecture is to replace the gravitational action by a nonlinear function of the Ricci scalar. The modified Friedmann equations can be derived in this description. Straumann further explains the chameleon mechanism and discusses the non-existence of relativistic stars in this theory.