The purpose of this mixed methods study was to explore the tensions between what secondary teachers think and believe about assessment and the corresponding actions they take in their classrooms. Of particular interest was the development of Teacher Assessment Identity (TAI) and the impact of experience on TAI. Survey data and semi-structured interview responses were analyzed and compared with existing research to determine how teachers assess, how TAI development is influenced by elements of the researcher's Practice -- Influential Contexts -- Conceptions -- Knowledge (PICK) model, and how TAI changes over the course of a teacher's career. Findings confirmed assessments based on need fulfillment, type, and processes of equity. In addition, the dynamic nature of TAI was found to be influenced prior to and during the secondary teacher's career by key external factors, including the teacher's assessment experiences as a student, desires for student success, the pandemic, and years of classroom experience. Tensions between participant conceptions and external influences were found to act as catalysts for change or barriers for TAI growth. Post hoc analysis showed differences in assessment practices based on standardized testing preparation. Recommendations for future research are presented as well as potential impact of the study to the field. [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.]