The International Association for the Evaluation of Educational Achievement's (IEA) mission is to enhance knowledge about education systems worldwide and to provide high-quality data that will support education reform and lead to better teaching and learning in schools. In pursuit of this aim, it conducts and reports on major studies of student achievement in literacy, mathematics, science, citizenship, and digital literacy. IEA studies, most notably Trends in International Mathematics and Science Study (TIMSS), Progress in International Reading Literacy Study (PIRLS), International Civic and Citizenship Education Study (ICCS), and International Computer and Information Literacy Study (ICILS), have set the benchmark for international comparative studies in education. These well-established studies have generated vast datasets encompassing student achievement, disaggregated in a variety of ways, along with a wealth of contextual information which contains considerable explanatory power. The numerous reports that have emerged from them represent an invaluable contribution to the corpus of educational research. Nevertheless, IEA's goal of supporting education reform needs something more: deep understanding of education systems and the many factors that bear on student learning advances through in-depth analysis of the global datasets. The IEA Research for Education series represents a further effort by IEA to capitalize on these unique datasets, so as to provide powerful information for policymakers and researchers. Each report focuses on a specific topic and is produced by a dedicated team of leading scholars on the theme in question. This fifth volume in the series is concerned with socioeconomic inequality and educational outcomes. Socioeconomic status is a key variable in social science research. It is especially important to the understanding of educational inequality and how best to address it. There is a substantial literature on the links between students' educational achievement and their family background. Despite this, challenges in measuring socioeconomic status and identifying its impact persist. This book draws on data collected over 20 years in the IEA Trends in International Maths and Science Study (TIMSS) and scrutinizes student achievement levels in relation to their socioeconomic status. Besides achievement data, TIMSS has been collecting background information from students, teachers, and school principals. Using a modified version of the TIMSS home educational resources index, the authors have identified tentative patterns in the changes over time. Specifically, they have established which countries have seen greater educational inequality attributable to family background and which have seen a reduction. They also identify which countries have managed to increase the academic performance of disadvantaged students over the period and those which have not. [For volume 4, see ED588363.]