This is the first introductory, undergraduate-level book published on diatonic set theory, an imprtant but often over-looked sub-field of music theory. It provides an easy entry point into this challenging field and is written at an appropriate level for introductory music majors, motivated non-majors, music theorists, and mathematicians with musical interests. Foundations of Diatonic Theory supplements the text for any Fundamentals of Music or Music Theory course, and is suitable for self-study of basic music theory in mathematical context.