I will discuss recent progress understanding how light particles thermalized in the early universe can be detected in the CMB ad large-scale structure surveys. We will focus in particular on the phase shift of the acoustic peaks caused by free streaming of these relics prior to recombination.  We will discuss the origin of this signal in detail including the robustness to nonlinear evolution in the late universe.  Finally, we will discuss the recent measurement of the cosmic neutrino background in BOSS DR12 from this phase shift and future applications of these types of analyses.