Design theory has grown to be a subject of considerable interest in mathematics, not only in itself, but for its connections to other fields such as geometry, group theory, graph theory and coding theory. This textbook, first published in 1985, is intended to be an accessible introduction to the subject for advanced undergraduate and beginning graduate students which should prepare them for research in design theory and its applications. The first four chapters of the book are designed to be the core of any course in the subject, while the remaining chapters can be utilized in more advanced or longer courses. The authors assume some knowledge of linear algebra for the first half of the book, but for the second half, students need further background in algebra.