For a long time, global fits of the electroweak sector of the Standard Model (SM) have been used to exploit measurements of electroweak precision observables at lepton colliders (LEP, SLC), together with measurements at hadron colliders (Tevatron, LHC), and accurate theoretical predictions at multi-loop level, to constrain the free parameters of the SM, like the Higgs and top masses. Today, all fundamental SM parameters, including information on the Higgs couplings, are experimentally determined and the global fits are used as powerful tools to assess the validity of the theory and to constrain scenarios for new physics. Future measurements at the Large Hadron Collider (LHC) and the International Linear Collider (ILC) promise to improve the experimental precision of key observables used in the fit. After an update of the current electroweak fit results based on latest 2-loop calculations, we present the prospects for the LHC and ILC. We perform a detailed analysis of the impact of present and future sources of systematic uncertainties on the fit predictions. By using LHC measurements of the signal strength of Higgs production and decay channels we examine the present and future impact of electroweak precision observables on a model with modified couplings of the Higgs boson to fermions and bosons.