Summary: Proportional reasoning is perhaps one of the most important types of mathematical thinking for elementary school students to develop. It includes aspects of rational numbers, spans the entire mathematics curriculum, and is a significant foundation for mathematical proficiency. Understanding studentsâ€™ use of proportional reasoning is a basis on which to develop benchmarks or guideposts that can provide a descriptive picture of the learning progression between elementary school math and mathematics encountered in later grades. As part of a school-based project focused on collecting information about proportional reasoning, fifth-grade mathematics students worked on several problems and then composed a problem to reflect a similar context. For students to construct their own problems, they must possess a rich understanding of the mathematical relationships in their work. Writing and sharing math problems moves students beyond the procedural understanding necessary to solve many types of problems presented in textbooks by promoting opportunities for students to personally grapple with important mathematics in an engaging, problem-posing environment. (Contains 3 figures.) (ERIC)