Constraining dark energy with and after GW170817

Requiring the luminal propagation of gravitational waves imposes strong constraints on theories of dark energy and modified gravity. I will review these constraints, highlighting a number of caveats that are particularly important in their physical interpretation. In addition I will argue that further theoretical considerations, e.g. from requiring radiative stability, allow us to place strong additional restrictions on departures from General Relativity. Throughout the talk I will also present cosmological parameter (MCMC) constraints using Planck, BAO, matter power spectra and RSD data, showing how these are significantly tightened by the above-mentioned theoretical requirements.