This thesis is about the process of becoming, or not becoming, a primary school mathematics teacher. The aim is to understand and describe the professional identity development of novice primary school mathematics teachers from the perspective of the novice teachers themselves. The study is a case study with an ethnographic direction where seven novice teachers have been followed from their graduation and two years onwards. The ethnographic direction has been used to make visible the whole process of identity development, both the individual and the social part. The empirical material in the study consists of self-recordings made by the respondents, observations and interviews. The empirical material is analysed in two different but co-operating ways. First a conceptual framework was developed and used as a lens. Second, methods inspired by grounded theory are used. The purpose of using them both is to retain the perspective of the respondents as far as possible. At the time of graduation the respondents are members in a community of reform mathematics teaching and they want to reform mathematics teaching in schools. In their visions they strive away from their own experiences of mathematics in school and practice periods. Four cases are presented closely in the thesis as they show four various routes into, and out of, the teaching profession. These four cases make visible that the respondents’ patterns of participation regarding teaching mathematics changes when they become members in new communities of practice with mathematics teaching as part of the shared repertoire. But, the four cases also make visible that the existence of such communities of practice seems to be rare and that the respondents’ different working conditions limit their possibilities of becoming members in those that exist. During the time span of this study, the respondents hardly receive any feedback for their performance as mathematics teachers. Even if they teach mathematics they don´t teach it as they would like to and they don´t think of themselves as mathematics teachers. Two years after graduation none of the respondents has developed a professional identity as primary school mathematics teacher. A primary school teacher in Sweden is a teacher of many subjects but they are the first teachers to teach our school children mathematics. For the respondents to develop a sense of themselves as a kind of primary school mathematics teacher, mathematics teaching has to become part of their teacher identities. For this to become possible, mathematics must become a part of their image of a primary school teacher as an image of a primary school mathematics teacher. Furthermore memberships in communities of practice with mathematics in the shared repertoire must be accessible, both during teacher education and after graduation. Then professional identity development as a primary school teacher would include becoming and being a teacher of mathematics.