Explore the epoch of reionization in this comprehensive lecture by T. R. Choudhury, part of the "Cosmology - The Next Decade" program at the International Centre for Theoretical Sciences. Delve into key topics including evidence for reionization through the Lyman-alpha forest, absorption signatures and spectra, the Gunn-Peterson effect, and OSO absorption lines. Learn about evolution equations, flux power spectrum calculations, and the CMBR angular power spectrum. Examine how the cosmic microwave background radiation can be used to probe reionization through Thomson scattering. Discuss observational constraints and challenges in studying this crucial period of cosmic history. Gain insights into the latest developments in cosmology and prepare for the next generation of experiments that will shed light on the Universe's dark components and reionization history.