In the book Theory of Principles and Applications, Siegfried Engelmann and co-author Douglas Carnine describe the theory underlying the development of Direct Instruction curriculums. Engelmann and Carnine not only spell out in detail the scientific and logical basis on which their theory is based, but provide a multitude of in-depth descriptions and guidelines for applying this theory to a wide range of curriculum. This book will help the reader understand why the Direct Instruction programs authored by Engelmann and his colleagues have proven uniquely effective with students from all social and economic backgrounds, and how the guidelines based on the theory can be applied to a wide range of instructional challenges, from designing curriculums for disadvantaged preschoolers to teaching algebraic concepts to older students.