This study addresses the effectiveness of a widely used core reading program that reflects the research-based practices recommended by the National Reading Panel. This and other similar programs are increasingly used to prevent reading difficulties and ensure that all children are reading at or above grade level by the end of third grade. Effective early reading instruction is critical for preventing later reading difficulties. With twothirds of 4th grade students failing to achieve proficiency in reading on the National Assessment of Educational Progress in 2013, the need to implement and test effective early reading programs is relevant and pressing. The "Open Court Reading" (OCR) program published by SRA/McGraw-Hill and widely used for almost 40 years is a phonics-based core-reading program for students in kindergarten to 6th grade that incorporates many of the instructional practices related to phonemic awareness, phonics, fluency, vocabulary, and text comprehension recommended by the National Reading Panel. In this study, an independent research team evaluated the effectiveness of the OCR program in a large national sample of elementary schools at scale, across diverse school populations and conditions, and with no more support than schools would have access to if they had selected OCR as their early reading curriculum apart from participation in a research project. The study participants included approximately 4,500 elementary school students and 1,200 teachers across 49 schools in 7 districts each year of the study. The evaluation of the OCR program involved two key elements: the multi-site cluster randomized trial (CRT) and the implementation study. Data from teachers and students in two cohorts (grades K&3 and grades 1&4) were gathered over two school years. Each year of data was collected as cross-sectional and each year is analyzed independently. This study provides preliminary evidence that the impacts of OCR are not significant on overall students' reading performance when implemented at scale in a large sample of schools after one or two years relative to other core reading curricula. However, there were positive differential impacts for Kindergarten students as well as Hispanic students. Tables are appended. [SREE documents are structured abstracts of SREE conference symposium, panel, and paper or poster submissions.]