The goals of LETRS are to build teachers' knowledge of language structure and the processes involved in learning to read words, spell, and comprehend, and then to help teachers apply these understandings in their classrooms. The LETRS courses, which are designed for teachers in grades K-3, are to be implemented over two years. In the first year of LETRS, teachers learn how to teach phoneme awareness, beginning and advanced decoding, word recognition, and spelling. In the second year, participants shift their focus to oral language, vocabulary, reading comprehension, and writing in response to reading. Several theoretical frameworks for understanding reading and writing in grades K-3 provide conceptual cohesion and are woven throughout the eight units. Each unit, however, allows teachers to focus on one important aspect of teaching at a time. The courses build knowledge in a progressive sequence in which one topic supports and is connected to the next--a feature that distinguishes LETRS from professional development offered as a patchwork of various options that teachers can self-select. In this article, Louisa C. Moats explains the origin and development of LETRS and the rationale for the course content.