The Foundational Handbook on Improvement Research in Education is a pathbreaking effort to build a field of research committed to producing the practical knowledge needed to advance educational access, quality, and equity. This is research distinguished by the use of inclusive, iterative approaches to analysis, design, implementation, and evaluation to understand and address educational opportunities, needs, and problems grounded deeply in school and community contexts. Designed for researchers, students, and educators, the handbook elaborates the intellectual foundations, explores the organizational and policy contexts, reviews approaches, and examines methods of improvement research. It features contributions from a plural community of researchers with expertise in the learning sciences, instructional improvement, organizational and policy studies, and research methodologies, many with extensive experience collaborating with teachers, leadership, families, and advocates in local problem solving and design.