IEA’s PIRLS (Progress in International Reading Literacy Study) is an ongoing international assessment program of students’ reading achievement in their fourth year of schooling - an important transition point in their development as readers. By this time in their education, students typically have learned how to read and are now reading to learn. Conducted every five years since 2001, PIRLS is recognized as the global standard for assessing trends in reading achievement at the fourth grade. PIRLS 2021 was the fifth assessment cycle. Successfully conducted in 57 countries and eight benchmarking entities, PIRLS 2021 differs from previous PIRLS assessments in several ways. First, a substantial creative effort was focused on transitioning PIRLS 2021 to an innovative digital assessment with 23 colorful and engaging texts delivered to students using a new group adaptive design. Second, PIRLS 2021 data collection occurred over two years during the unprecedented COVID-19 pandemic. Although collecting data in schools faced many disruptions, most countries met the standards for high-quality data collection. This tremendous effort resulted in PIRLS 2021 providing the only internationally comparative fourth grade achievement results collected during the pandemic.