The purpose of this qualitative phenomenological research study was to explore and illuminate lived professional developmental experiences and personal perspectives of a select group of K-2 mathematics teachers. According to Creswell and Poth (2017), researchers collect and analyze data from people who experience a phenomenon to describe "the universal essence" of the phenomenon (p. 75). The findings from this study add to the literature and provide insight into teachers' experiences and perceptions of their mathematics professional development. The study implications suggest advanced professional development formats and platforms related to adult learning include as-needed relevant individualized mathematics professional development experiences that may increase teacher quality, deepen student foundational mathematical development, and improve student learning outcomes in 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/en-US/products/dissertations/individuals.shtml.]