"First Light in the Universe" is a scholarly book that explores the first billion years of the universe's history, starting from the so-called Dark Ages to cosmic reionization. This phase represents one of the greatest challenges in contemporary astrophysics and is a central driving force for future observational facilities. The book is based on detailed notes from lectures delivered by three renowned scientists during the 36th Saas-Fee Advanced Course: Abraham Loeb, Andrea Ferrara, and Richard Ellis. The topics covered include the formation of the first stars and black holes, the initial mass function, feedback effects, early dust formation, the history of cosmic star formation, distant galaxies, cosmic reionization, and the cosmic infrared background. This book provides an accessible and up-to-date overview of the field and is aimed at graduate students in astronomy, cosmologists, physicists, and researchers.