  After recombination the cosmic gas was left in a cold and neutral state.
However, as the first stars and black holes formed within early galactic
systems, their UV and X-ray radiation induced a gradual phase transition of the
intergalactic gas into the warm and ionized state we currently observe. This
process is known as cosmic reionization. Understanding how the energy
deposition connected with galaxy and star formation shaped the properties of
the intergalactic gas is one of the primary goals of present-day cosmology. In
addition, reionization back reacts on galaxy evolution, determining many of the
properties of the high-redshift galaxy population that represent the current
frontier of our discovery of the cosmos. In these two Lectures we provide a
pedagogical overview of cosmic reionization and intergalactic medium and of
some of the open questions in these fields.
