Coherence and Sakharov Oscillations in the Microwave Sky

I discuss the origin of the Sakharov oscillations'' (or secondary Doppler peaks’‘) in standard angular power spectra of the Cosmic Microwave Background anisotropies calculated for inflationary models. I contrast this with the production of CMB anisotropies in `active’ models and argue that the presence or absence of Sakharov oscillations will reveal fundamental properties of the primordial perturbations.