A process in the early Universe is in thermal equilibrium, only if the rate per particle is much bigger than the Hubble parameter. This is because a typical particle in the opposite case would not interact within a Hubble time, which because of the expansion of the Universe means that it would not interact at all. In this chapter I describe thermal equilibrium in general, and then see how it applies in the early Universe. I will use natural units, and set the Boltzmann constant to 1 so that T is measured in eV.