Contents: Part I:Overview. T.P. Carpenter, E. Fennema, T.A. Romberg, Toward A Unified Discipline of Scientific Inquiry. Part II:Content Analyses of Rational Numbers. M.J. Behr, G. Harel, T. Post, R. Lesh, Rational Numbers: Toward a Semantic Analysis -- Emphasis on the Operator Construct. T.E. Kieren, Rational and Fractional Numbers: From Quotient Fields to Recursive Understanding. Part III:Students' Thinking. N.K. Mack, Learning Rational Numbers With Understanding: The Case of Informal Knowledge. L.B. Resnick, J.A. Singer, Protoquantitative Origins of Ratio Reasoning. S.J. Lamon, Ratio and Proportion: Children's Cognitive and Metacognitive Processes. Part IV:Teachers and Teaching. D.L. Ball, Halves, Pieces, and Twoths: Constructing and Using Representational Contexts in Teaching Fractions. C.A. Brown, A Critical Analysis of Teaching Rational Number. J.A. Hiebert, Benefits and Costs of Research That Links Teaching and Learning Mathematics. J.T. Sowder, N. Bezuk, L.K. Sowder, Using Principles From Cognitive Psychology to Guide Rational Number Instruction for Prospective Teachers. Part V:Assessment. S.P. Marshall, Assessment of Rational Number Understanding: A Schema-Based Approach. Part VI:Curriculum and Instruction. L. Streefland, Fractions: A Realistic Approach. T.R. Post, K.A. Cramer, M. Behr, R. Lesh, G. Harel, Curriculum Implications of Research on the Learning, Teaching, and Assessing of Rational Number Concepts.