Summary: Astrobiology, like many (but not all) sciences, must take into account questions of the “Why?”, “Where?”, “How?” and “When?” type. In this introductory chapter, we explain why, in this book, we will only consider two of these questions that are, moreover, deeply interrelated. Chronology is by definition related to the “when?” question but as soon as we are interested in the history of Earth or the history of life, it is impossible to treat these questions and their answers without explicit references to the “how?” questions.