Abstract. Four-dimensional field theories with extended supersymmetry have represented an important theoretical laboratory for the exploration of the nonperturbative dynamics since the discovery of the Seiberg-Witten solution, but a general method for constructing and classifying them is still missing. In this talk I will introduce a new geometric method called S-fold, which denotes F-theory compactifications involving non-trivial S-duality transformations, to construct strongly-coupled N=2 theories in four dimensions. This allows us to define a new infinite family of superconformal theories and to recover all models with one-dimensional Coulomb branch. The resulting field theories are connected by an interesting web of RG flows and their Higgs branches provide new examples of instanton moduli spaces.