Because of the educational importance of mathematics and science, IEA's (International Association for the Evaluation of Educational Achievement) Trends in International Mathematics and Science Study, widely known as TIMSS, is dedicated to providing countries with information to improve teaching and learning in these curriculum areas. Conducted every four years on a regular cycle, TIMSS assesses achievement in mathematics and science at the fourth and eighth grades. TIMSS 2011 is the most recent in the TIMSS series, which began with the first assessment in 1995 and has continued with subsequent assessments in 1999, 2003, and 2007. For countries with data back to 1995, TIMSS 2011 will provide the fifth trend measure. This publication contains three frameworks and explains the assessment design that will serve as the basis for implementing TIMSS 2011. The "TIMSS 2011 Mathematics Framework" and the "TIMSS 2011 Science Framework" in Chapters 1 and 2, 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, etc. in mathematics, and biology, chemistry, etc. 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 describing the thinking students should be doing within the mathematics and science content domains are the same for mathematics and science and parallel across grades, but with different levels of emphasis. Chapter 3 contains the "TIMSS 2011 Contextual Framework" describing the types of situations and factors associated with students' learning in mathematics and science that will be investigated via the questionnaires. Finally, Chapter 4 provides an overview of the "TIMSS 2011 Assessment Design", including general parameters for item development. Appendices include: (1) Acknowledgements; (2) Example Mathematics Items; and (3) Example Science Items. (Contains 12 exhibits and 6 footnotes.) [Funding for this paper was provided by the Boston College.