Currently, studies on STEAM education or arts integration focus on students' experiences rather than teachers' challenges. Thus, the purpose of this dissertation is to set an example for policymakers, educators, and researchers, to provide better support for art-integrated mathematics and biology teachers by understanding their professional identity growth and their informal art learning. In this qualitative study, I employ case study as my research methodology by conducting interviews, observation, and physical artifacts. My sample consists of a high school mathematics teacher and a biology teacher who bring art to the teaching. My research findings highlight the significance of artistic thinking for art-integrated teachers' teaching. The implications of the study indicate that art-integrated teachers' artistic thinking can be stimulated by enjoying art and expanding the comprehension of art rather than only depending on art specialists. [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.]