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This technical report documents the methods and procedures used to develop, implement, and report the results of PIRLS 2021. PIRLS 2021 incorporated two major advances in international reading assessment at the fourth grade, including transitioning from paper to digital assessment and implementing a group adaptive assessment design. There were substantial challenges in the implementation, analysis, and reporting of PIRLS 2021 due to the COVID-19 pandemic. PIRLS 2021 data collection occurred over a two-year period, with schools worldwide facing many disruptions to normal instruction. Despite many obstacles, most countries met standards for high quality data. A multi-step, population-based linking approach to the achievement scaling enabled reporting comparable trends for paper and digital data on the PIRLS trend scale. [Publisher summary, ed]