Not every effective theory of fields and gravitational interactions at low energy can be embedded into a consistent theory of gravity at short distances. I will discuss how fundamental properties of scattering amplitudes, such as unitarity and causality of the underlying fundamental theory, can be used to spot inconsistent theories of gravity at low-energy, and hence throw them in the so-called EFT ``swampland'’. I will focus on two main applications: the weak gravity conjecture for the physics of extremal black holes; and modified gravity theories such as Galileons, of interest for late-time cosmology.