Ralph W. Tyler is best known for Basic Principles of Curriculum and Instruction, his syllabus for Education 360 at the University of Chicago published in 1949. The significance of this text is considered far-reaching; it ranks as one of the most influential writings in curriculum development. This often-criticized problem solving rationale for curriculum development has elicited debate within the curriculum field for the past three decades, even to the point of some scholars calling for a reconceptualization of the field. The persistent criticism of Tylers Rationale indicates the continuing importance of his 1949 book to the curriculum field.This study describes the origins, features, and major interpretations of the Tyler Rationale, explores the similarities and differences between the Tyler Rationale and Tylers unfinished 1970s Revision, and considers how Tylers 1970s Revision can help us understand Tylers Rationale. This historical study utilizes Ralph Tyler Project archival documents at the University of Chicago in the Ralph Tyler Project collection, which include Tylers 1970s Revision to Basic Principles of Curriculum and Instruction. A comparative analysis of Tylers original 1949 Rationale and Tylers 1970s Revision describes similarities between these texts, and also identifies clarifications and additions Tyler made to his thinking about curriculum development in the 1970s Revision. Some but not all of these clarifications and additions may have been in response to extant criticisms of his 1949 Rationale.As part of a funded effort Tyler drafted a preface and six chapters, which elaborated chapter one of his 1949 Rationale. In the drafted chapters, Tyler clarified some aspects of the 1949 Rationale that had come under criticism, including the change of linearity of the four fundamental questions, using the learner, subject matter and contemporary life as sources for deriving objectives, and the use of philosophy and psychology as means for screening educational objectives. The most notable change in Tylers proposals for curriculum development was a greater emphasis on the learner as a source for deriving educational purposes. This change indicates Tylers increased commitment to the active participation of the student in the educational process.