In this introductory chapter, the subject of mathematical explanation of empirical phenomena is positioned in relation to the topics of scientific explanation and mathematical explanation. This serves as the stepping stone towards the main matter of the book. Part one, The Indispensability of Mathematics, pertains to the role of mathematical entities in mathematical explanations of empirical phenomena. Part two, Mathematical Counterfactual Dependence, pertains to the topic of mathematical explanation within the framework of a counterfactual theory of explanation.