In this chapter the description of the early universe to cover the first several hundred thousand years of its existence will be presented. This leads to one of the most important pillars of the Big Bang model: the cosmic microwave background radiation or CMB. It will be seen how and when the CMB was formed and what its properties are. Most important among these are its blackbody energy spectrum characterized by an average temperature of \(T = 2.725\,\mathrm{K}\), and the fact that one sees very nearly the same temperature independent of direction.