As states and districts overhaul the way their schools teach reading, many are banking on one specific professional-learning program to propel this transformation: Language Essentials for Teachers of Reading and Spelling, commonly known as LETRS.  LETRS instructs teachers in structured literacy—sound-letter sequences, language structure, comprehension, and vocabulary—and why and how to plan to teach them. And it delves into the research base behind these recommendations. The program is long, intensive, and expensive.  Twenty-three states have contracted with Lexia, the company that houses LETRS, to provide some level of statewide training, and nearly 200,000 teachers total are enrolled in the training this year. How did this one training become so ubiquitous? Is it really that different from the other PD options? And most importantly: Does it work?