Middle school students consolidate their understanding of integers and rational numbers, increasing their facility with fractions, decimals, and percents and encountering proportionality. This book shows how students can explore these important ideas in such diverse activities as exchanging currency and using area models to develop algorithms for multiplying and dividing fractions. Underlying each activity is the belief that students learn best when they build computational fluency on a firm conceptual foundation. Numbers and the operations that we perform with them are the starting points for all mathematics. Accordingly, "Principles and Standards for School Mathematics" makes the "Number and Operations Standard" first among the five Content Standards and gives it a centrality across the mathematics curriculum from prekindergarten through grade 12. Following an Introduction, this book is divided into three chapters: (1) Fractions, Decimals, and Percents; (2) Working with Fractions, Decimals, and Percents; and (3) Proportional Reasoning. Following Chapter 3 is a section titled "Looking Back and Looking Ahead." An appendix titled "Blackline Masters and Solutions" and a list of references conclude the book. An accompanying CD-ROM contains: (1) Table of Standards and Expectations, Number and Operations, Pre-K-12; (2) Applets; (3) Blackline Masters; and (4) Readings from Publications of the National Council of Teachers of Mathematics.