  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.
