The "TIMSS 2019 Assessment Frameworks" provides the foundation for the four international assessments that comprise the International Association for the Evaluation of Educational Achievement (IEA's) TIMSS (Trends in International Mathematics and Science Study) 2019: TIMSS Mathematics--Fourth Grade, TIMSS Mathematics--Eighth Grade, TIMSS Science--Fourth Grade, and TIMSS Science--Eighth Grade. The report begins with an introduction by Ina V. S. Mullis. The "TIMSS 2019 Mathematics Framework" and the "TIMSS 2019 Science Framework" presented in Chapters 1 (Mary Lindquist, Ray Philpot, Ina V.S. Mullis, and Kerry E. Cotter) and 2 (Victoria A.S. Centurino and Lee R. Jones), respectively, describe in some detail the major content and cognitive domains in mathematics and science to be tested at the fourth and eighth grades. The content domains (for example, algebra, geometry in mathematics, and biology, chemistry in science) and the topic areas within the domains are described separately for the fourth and eighth grades with each topic area elaborated with specific objectives. The cognitive domains of knowing, applying, and reasoning describe the thinking skills students are expected to use as they engage with the mathematics and science content. Chapter 3 (Martin Hooper, Ina V.S. Mullis, Michael O. Martin, and Bethany Fishbein) contains the TIMSS 2019 Context Questionnaire Framework describing the home, school, and classroom factors associated with students' learning in mathematics and science that will be investigated via the questionnaires completed by students, their parents, teachers, and school principals. Finally, Chapter 4 (Michael O. Martin, Ina V.S. Mullis, and Pierre Foy) provides an overview of the assessment design including general parameters for item development. Appendix A contains Acknowledgments and Appendix B presents Example Restricted Use Items.