Conducted every five years, Progress in International Reading Literacy Study (PIRLS) assesses international trends in the reading comprehension of young students in their fourth year of schooling--an important transition point in children's development as readers. PIRLS was designed to complement the International Association for the Evaluation of Educational Achievement's TIMSS assessments of mathematics and science at the fourth grade. The first two chapters of the PIRLS 2021 Assessment Frameworks contain the PIRLS 2021 Reading Framework and PIRLS 2021 Context Questionnaire Framework, respectively. The reading framework provides guidelines for assessing reading comprehension at the fourth grade according to a matrix of two reading purposes--literary and informational--by four comprehension strategies (retrieval, inferencing, integrating, and evaluation). The context questionnaire framework describes the topics to be covered by the PIRLS 2021 questionnaires. The third chapter describes the assessment design for PIRLS 2021. Across the past 20 years, linking the PIRLS assessments of reading achievement to the rich array of PIRLS questionnaire data about the contexts in which students learn to read has provided researchers an important source of policy relevant information about how to improve reading education around the world. [For "PIRLS 2016 Assessment Framework. 2nd Edition," see ED545242.]