This yearbook sharpens the views of mathematical reasoning and its development at all grade levels. Various perspectives about the nature of reasoning and addresses the many issues and concerns involving mathematical reasoning such as how learners reason in mathematics, how communication promotes reasoning, how teachers gather evidence of student reasoning, what curricular approaches can be profitably explored, and what can be done to ensure success in developing reasoning are presented. Chapters include: (1) "Mathematical Reasoning in the Elementary Grades" (Susan Jo Russell); (2) "Developing Mathematical Reasoning in the Middle Grades: Recognizing Diversity" (Carol E. Malloy); (3) "Reasoning by Analogy: A Fundamental Process in Children's Mathematical Learning" (Lyn D. English); (4) "The Nature of Mathematical Reasoning" (Robert J. Sternberg); (5) "Young Children's Mathematical Reasoning: A Psychological View" (Eileen P. Tang and Herbert P. Ginsburg); (6) "Reasoning about Operations: Early Algebraic Thinking in Grades K-6" (Deborah Schifter); (7) "Teaching Fractions: Fostering Children's Own Reasoning" (Constance Kamii and Mary Ann Warrington); (8) "Developing Mathematical Reasoning within the Context of Measurement" (Kay McClain, Paul Cobb, Koeno Gravemeijer, and Beth Estes); (9) "Mathematics Is for the Birds: Reasoning for a Reason" (Phyllis Whitin and David Whitin); (10) "Mathematical Reasoning during Small-Group Problem Solving" (Alice F. Artzt and Shirel Yaloz-Femia); (11) "Developing Students' Algebraic Reasoning Abilities" (Carole Greenes and Carol Findell); (12) "Innovative Tasks to Improve Critical--and Creative--Thinking Skills" (Stephen Krulik and Jesse A. Rudnick); (13) "Understanding Students' Probabilistic Reasoning" (Graham A. Jones, Carol A. Thornton, Cynthia W. Langrall, and James E. Tarr); (14) "Analyzing Mathematical Reasoning in Students' Responses across Multiple Performance Assessment Tasks" (Dominic Peressini and Norman Webb); (15) "Mathematical Reasoning: In the Eye of the Beholder" (Peggy House); (16) "The Language of Quantification in Mathematics Instruction" (Susanna S. Epp); (17) "Mathematics Journal Articles: Anchors for the Guided Development and Practice of Reasoning Skills" (Robert Gerver); (18) "Teaching and Assessing Statistical Reasoning" (Joan B. Garfield and Iddo Gal); (19) "Mathematical Reasoning and Educational Policy: Moving Beyond the Politics of Dead Language" (William F. Tate and Howard C. Johnson); (20) "Adventures in Mathematical Inquiry" (Gail Boldt and Aaron Levine); (21) "Using Short Questions to Develop and Assess Reasoning" (William M. Carroll); (22) "Helping Elementary and Middle Grades Preservice Teachers Understand and Develop Mathematical Reasoning" (George W. Bright); and (23) "Twenty Questions about Mathematical Reasoning" (Lynn Arthur Steen). (ASK)