A strong sense of identity and belonging in different communities can help mathematics teachers have more confidence in their first years of teaching and help their students have positive experiences in the mathematics classroom as well. My qualitative study sought to investigate identity formation within the context of preservice and early career secondary mathematics teachers to understand their identities and definitions within each community as well as what factors influenced their formation of those identities and definitions. I was guided by communities of practice to frame identity formation as a sense of belonging to specific communities. The communities I focus on in relation to mathematics teachers are the communities of mathematicians, teachers, and mathematics teachers. In my study, ten participants who completed an autoethnography during a secondary mathematics methods course in their teacher preparation program, completed an interview between two to five years after this course. I asked participants to actively engage in reflection on their autoethnography and on new experiences related to their identity formation as mathematics teachers. During the interviews, participants discussed different experiences, research, and their own beliefs in relation to each of the communities of mathematicians, teachers, and mathematics teachers. Participants reflected on their own lived experiences to describe and define themselves and the communities. Definitions for each community were different among all participants as they negotiated their roles and sense of belonging to those communities. Though all participants started their journey in the same teacher preparation program ready to become mathematics teachers, they all discussed the idea of a mathematics teacher differently and placed themselves on a scale from mathematician to teacher in different positions on that scale. While a mathematics teacher may need to know mathematics and know how to teach, those are not the only things that make a mathematics teacher. Institutional, discourse, and affinity factors all played a part in each of these participants' formation of identity and definitions. From these influences, participants had to negotiate tensions between what they believed and what they learned from their lived experiences. Identity formation is different for each person and as teacher become aware of their own identities, they are able to have more confidence in their own teaching practices and positively impact their students in the classroom as well. Further research can help identify ways in which allowing teachers to create their own definitions and views of these communities could increase a sense of belonging in the profession of mathematics teachers. Professional development opportunities can also be provided for mathematics teachers to help them take an active role in forming their identity, understanding how they fit into the communities around them, and help them have a positive impact on their students' beliefs of themselves in the mathematics classroom. [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.]