Abstract. After reviewing some motivation and the relevant parameter space, I will illustrate the impact of massive sterile neutrinos (tens or hundreds of MeV) on core collapse supernovae. An introduction to the topic of neutrinos in supernovae will be included for those unfamiliar with the relevant astrophysics. In particular, I will discuss constraints in the mass-mixing plane and interesting phenomenological signatures measurable at existing or forthcoming neutrino detectors. Cosmological probes (primordial nucleosynthesis, CMB) are also sensitive to some allowed parameter space. I will present recent calculations of the thermal decoupling of these states and the impact on the observables N_eff and Y_p . Current constraints and forecasts for the reach of the Stage-IV CMB observations will be discussed.