In this talk I will describe some recent progress in the study of 6d and 4d effective actions of F-theory. Our approach uses the duality of F-theory to M-theory in one dimension lower. I will show how 5d and 3d Chern-Simons terms play a crucial role in determining the spectrum and anomalies of the higher-dimensional theory. This lower-dimensional point of view also allows for an interesting perspective on formulating 6d (2,0) tensor theories. If time permits I will comment on 6d symmetry-breaking vacua of gauged supergravities in F-theory.