This open access book discusses challenges in school improvement research and different methodological approaches that have the potential to foster school improvement research. Research on school improvement and accountability analysis places high demands on a study's design and method. The potential of combining the depth of case studies with the breath of quantitative measures and analyses in a mixed-methods design seems very promising. Consequently, the focus of the book lies on innovative methodological approaches. The book chapters address design, measurement, and analysis developments as well as theoretical and conceptual developments. The relevance of the research presented in the chapters for educational accountability is discussed in the book's discussion chapter. More specifically, authors present one specific innovative methodological approach and clarify that approach with a concrete example in the context of school improvement, based on empirical data when possible.This book series intends to bring together an array of theoretical and empirical research into accountability systems, external and internal evaluation, educational improvement, and their impact on teaching, learning and achievement of the students in a multilevel context. The series will address how different types of accountability and evaluation systems (e.g. school inspections, test-based accountability, merit pay, internal evaluations, peer review) have an impact (both intended and unintended) on educational improvement, particularly of education systems, schools, and teachers. The series addresses questions on the impact of different types of evaluation and accountability systems on equal opportunities in education, school improvement and teaching and learning in the classroom, and methods to study these questions. Theoretical foundations of educational improvement, accountability and evaluation systems will specifically be addressed (e.g. principal-agent theory, rational choice theory, cybernetics, goal setting theory, institutionalisation) to enhance our understanding of the mechanisms and processes underlying improvement through different types of (both external and internal) evaluation and accountability systems, and the context in which different types of evaluation are effective. These topics will be relevant for researchers studying the effects of such systems as well as for both practitioners and policy-makers who are in charge of the design of evaluation systems.