The reionization era refers to a period in the history of the universe, roughly between 150 million and one billion years after the Big Bang, when neutral hydrogen gas that filled the universe was ionized by the first stars and galaxies. This process marked the transition from a mostly neutral to a mostly ionized universe, allowing light to travel freely through space and enabling the universe to become transparent. Studies of the reionization era seek to understand the astrophysical processes that drove this transition, the properties of the first stars and galaxies, and the impact of reionization on the evolution of the universe. Observations of distant galaxies, quasars, and the cosmic microwave background radiation provide valuable insights into this important period in cosmic history.