I review the present knowledge of two fundamental processes in cosmology: galaxy formation and reionisation. After a brief description of the observations informing on reionisation, I then focus on the theoretical modelling of galaxy formation and the evolution of the intergalactic medium.I show that all the information we currently have on cosmic evolution at high-z combined with the preceding one on reionisation allows us to severely constrain the reionisation process. Specialattention is paid to the possible implications on that scheme of some recent unexpected observations.