Abstract. In this talk I will introduce the main motivations to explore theories of gravity alternative to General Relativity. In particular, I will focus on the popular class of scalar-tensor theories, discussing their formulation and how to parametrize them efficiently on a cosmological background. This framework is optimal to describe large class of models at once, which makes it ideal to test deviations from the standard cosmological model. I will discuss some of the main results in the literature, as well as current constraints on the parameters. Finally, I will speculate about the future of alternative theories of gravity and show what we can realistically expect from future surveys.