  The universe goes through several phase transitions during its formative
stages. Cosmic reionization is the last of them, where ultraviolet and X-ray
radiation escape from the first generations of galaxies heating and ionizing
their surroundings and subsequently the entire intergalactic medium. There is
strong observational evidence that cosmic reionization ended approximately one
billion years after the Big Bang, but there are still uncertainties that will
be clarified with upcoming optical, infrared, and radio facilities in the next
decade. This article gives an introduction to the theoretical and observational
aspects of cosmic reionization and discusses their role in our understanding of
early galaxy formation and cosmology.
