Fractions are foundational for success in algebra and STEM, but they are notoriously difficult for students. In past work, two separate branches of research from psychology and math education have studied fractions knowledge using different, isolated approaches. In this dissertation, I synthesize research from both fields about proportional reasoning, including the development of fractions knowledge. I then present two empirical studies that aim to bridge methods and theoretical perspectives from both fields to: 1. Compare three instructional approaches to support fraction magnitude knowledge in 2nd- and 3rd-grade students. 2. Test the degree to which common psychology and math education measures of fractions knowledge relate to each other and to 8th-grade students' algebra knowledge. Results of Study 1 show that a 15-minute lesson using proportional analogies on number lines (e.g., 3:5 :: 3/5:1) was effective for teaching students about fraction sizes. Results of Study 2 shed new light on the mechanisms that may drive the connections between fractions and algebra knowledge. Overall, this project suggests that interdisciplinary collaboration is necessary to better understand student learning and support student outcomes. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]