An overview is given of the non-equilibrium behaviour conceivable during the first million years of the Universe’s life. We describe various classical and quantum processes of this type which might constrain and fashion the irregularity spectrum on large cosmic scales. The likely fate of adiabatic, isothermal and vortical fluctuations is traced through the radiation dominated phase of a hot, big-bang Universe. Both the linear and nonlinear behaviour of these fluctuations is discussed and it is indicated how different preferred mass scales and irregularity spectra might be expected to arise in association with them . * Singularity is almost invariably a clue. The more featureless and commonplace a crime is, the more difficult . . .’ (Sherlock Holmes: The Boscombe Valley mystery , by Arthur Conan Doyle).