Abstract:Â Rapidly Oscillating Massive Particles (ROMPs) occur in quantum systems with non-diagonal Hamiltonians. The misalignment between flavor and mass eigenstates leads to oscillations between flavor states, such as those between electron and muon neutrinos in the Standard Model, or between active and sterile neutrinos in Beyond the Standard Model frameworks to name just a few examples. In this talk, I will discuss the general framework for particle production via oscillations. Of particular interest are ROMP systems where one flavor state, chi, is weakly coupled to the Standard Model (and hence a good dark matter candidate) and the second flavor state, psi, is coupled to the Standard Model such that psi oscillations to chi in the early universe can efficiently produce chi as the dark matter. I will discuss how oscillations, scatterings, thermal masses, and resonances all play a role to give ROMPs a rich cosmological story.