This textbook offers a comprehensive yet concise introduction to the theory behind price indices, covering the basics, historical background, necessary mathematical foundations, and contemporary cutting-edge theories. For those looking to study price indices, the most commonly used resources have traditionally been the comprehensive but voluminous CPI Manual, which is aimed at statistical practitioners, or Balk (2008), which, while covering theory concisely, requires advanced mathematical skills. This book is distinctive in that it (1) places emphasis on the relationship between price indices and economic theory, (2) presents cutting-edge methods and topics such as multilateral indices using scanner data and price measurement using Engel curves, and (3) provides explanations of essential mathematical techniques like functional equations, thus enabling self-study from the fundamentals to the forefront of modern index number theory.