This yearbook responds to the growing recognition among mathematics educators of the importance of teaching and learning estimation. It makes a strong case for the importance of estimation in such domains as numerosity, computation, and measurement. An important theme is the interdependency between the process of estimating in a particular domain and the understanding of mathematical concepts within that domain. The yearbook is divided into five sections The first five articles provide the framework, emphasizing the usefulnes of estimation, necessary thinking strategies and mind-set, the need for mental computation as a corequisite for computational estimation, and the arithmetic of approximate numbers. Part 2 (articles 6-l4) presents specific instructional activities, providing suggestions for teaching mental computation and different types of estimation in the primary grades and in the crucial areas of fractions, decimals, and percents. Part 3 (articles l5-2l) stresses the variety of types and uses for estimation. Part 4 (articles 22-25) focuses on estimation in measurement, including teaching activities and mathematical topics for grades K-12. Finally, part 5 (articles 26 and 27) comprises a review of research on teaching and learning estimation and a discussion of some difficulties and recent progress in attempts to test students' ability to estimate. (MNS)