  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.
