This qualitative research study explores teachers' understandings of multiplicative reasoning as a key developmental understanding (KDU). A KDU entails knowingly applying the same mathematical concepts within different contexts. A KDU supports an individual to build a connected understanding of mathematics as opposed to only understanding disconnected rules and procedures. In this study the KDU of multiplicative reasoning is defined and the researcher illustrates how this understanding connects the mathematical contexts of the elementary school curriculum. The data used to analyze the teachers' understandings of multiplicative reasoning was collected in a mathematics content course that focused on the KDU of multiplicative reasoning. The teachers also participated in a semi-structured interview the year following the course. The analysis of the data revealed that teachers did not continue to use the KDU of multiplicative reasoning once they were teaching, as they had during the course. If teachers are expected to teach a more connected understanding of mathematics as outlined in the Common Core State Standards then there needs to be more support for them to develop these understandings. Focusing on the development of KDUs is one way to begin to support these changes. This dissertation discusses how teachers may begin to develop an understanding of the KDU of multiplicative reasoning, which could allow them to teach a more connected understanding of mathematics. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]