  In this introductory chapter, we outline expectations for when and how the
hydrogen and helium atoms in the universe turned from neutral to ionized,
focusing on the earliest, least well understood stages, and emphasizing the
most important open questions. We include a historical summary, and highlight
the role of reionization as one of the few milestones in the evolution of the
universe since the Big Bang, and its status as a unique probe of the beginning
stages of structure formation.
