Scalar singlets at present and future colliders

A scalar singlet, coupled to the other particles only through its mixing with the Higgs boson, appears in several motivated extensions of the Standard Model. The prospects for the discovery of a generic singlet at the various stages of the LHC, as well as at future high-energy colliders, are studied, and the reach of direct searches is compared with the precision attainable with Higgs couplings measurements. The results are then applied to the NMSSM and Twin Higgs.