Over the past thirty years, research on teachers’ mathematical knowledge for teaching (MKT) has developed and grown in popularity as an area of focus for improving mathematics teaching and students’ learning. Many scholars have investigated types of knowledge teachers use when teaching and the relationship between teacher knowledge and student performance. However, few researchers have studied the sources of teachers’ pedagogical decisions and actions and some studies have reported that advances in teachers’ mathematical meanings does not necessarily lead to a teacher conveying strong meanings to students. It has also been reported that a teacher’s ways of thinking about teaching an idea and actions to decenter can influence the teacher’s interactions with students.This document presents three papers detailing a multiple-case study that constitutes my dissertation. The first paper reviews the constructs researchers have used to investigate teachers’ knowledge base. This paper also provides a characterization of the first case’s mathematical meaning for teaching angle measure and the impact of her meaning on her interactions with students while teaching her angle measure lessons. The second paper examines another instructor’s meaning for an angle and its measure and illustrates the symbiotic relationship between the teacher’s mathematical meanings for teaching and decentering actions. This paper also characterizes how an instructor’s commitment to quantitative reasoning influences the teacher’s instructional orientation and instructional actions. Finally, the third paper includes a cross-case analysis of the two instructors’ mathematical meanings for teaching sine function and their enacted teaching practices, including their choice of tasks, interactions with students, and explanations while teaching their sine function lessons.

Eleven years after being put into practice, the Common Core State Standards for Mathematics still take a back seat as traditional approaches drive many secondary geometry classrooms, specifically in regard to congruence. This thesis explores how university students reason about congruence based on their high school learning experience, as well as how in-service geometry teachers reason about and teach congruence. During the Summer of 2020, two distinct surveys were distributed to 33 undergraduate students at Arizona State University and two in-service geometry teachers in Arizona to characterize the ways they understand congruence and reflect on their experiences in secondary geometry classrooms. The results of the survey indicate that students who understood congruence either in terms of corresponding measurements or transformations were successful in identifying congruent shapes, while only students who understood congruence in terms of transformations were successful in constructing congruent shapes. Transformational reasoning was both the most productive and the least prominent way of understanding congruence among students. Their responses to activities and reflections on their experiences also suggested that deductive reasoning is not practiced or prioritized in many secondary geometry classrooms. Teacher understandings of congruence varied, and reflections suggested that development of materials and training that are aligned with the goals of CCSSM for both pre-service and in-service teachers would help teachers create an environment conducive to a transformational understanding of congruence and that promotes deductive reasoning.