The third edition of "The Theory of Interest" is significantly revised and expanded from previous editions. The text covers the basic mathematical theory of interest as traditionally developed. This book is a thorough treatment of the mathematical theory and practical applications of compound interest, or mathematics of finance. The pedagogical approach of the second edition has been retained in the third edition. The textbook narrative emphasizes both the importance of conceptual understanding and the ability to apply the techniques to practical problems. The third edition has considerable updates that make this book relevant to students in this course area.