We discuss the current status of theoretical and experimental constraints on a Higgs singlet extension of the Standard Model. We focus on the case of a real Higgs singlet extension with the second neutral Higgs boson being in the mass range accessible to the LHC (0 GeV - 1TeV). We consider constraints from perturbative unitarity, electroweak oblique parameters, perturbativity of the couplings, and vacuum stability, where the latter are tested up to a scale of 10^11 GeV using renormalization group equations. Furthermore, direct collider constraints from signal strength measurements and 95% C:L: exclusion limits from Higgs searches are included via the public codes HiggsSignals and HiggsBounds, respectively.We present predictions for the 14 TeV LHC run which are in accordance with all above constraints.