This thesis aims to explore the mathematics achievement of second language immigrants in compulsory school as they continue their schooling in Sweden. Specifically, the thesis aims to generate more knowledge about different sub-categories of second language students, namely newly arrived immigrants, early arrived immigrants and other second language students in compulsory school. The data in this thesis consists of students’ responses to test items and thus mainly contains mathematical symbols, essentially numbers in different representations, written by the students. Doing so, this thesis problematizes the concept of second language students in mathematics in two aspects. One aspect is to assess the first and second language students’ achievement in different mathematical content domains, instead of only assessing the total achievement. Another aspect is to see the second language students as different sub-categories of second language students. Papers I and II of this thesis found that the achievement difference between first and second language students is not homogeneous. Instead the achievement difference between first and second language students is larger for concepts that are rare in mathematics textbooks. Moreover, the achievement difference between first and second language students varies with the content domain. Another way to say this is that first and second language students have different achievement profiles. Papers III and IV of this thesis explored how sub-categories of second language students achieved on mathematics test items. Mathematics achievement studies on second language students often classify the second language students into a single category of students. Methodologically this imposes a concept of viewing second language students as homogeneous in proficiency in the language of instruction. This view is challenged in this thesis by dividing the second language students into newly arrived immigrants, early arrived immigrants and other second language students. These three sub-categories have different proficiency in Swedish language due to how long they have lived in Sweden. Papers III and IV found that these student categories both had different test achievement and, related to this, also used mathematical concept representations differently. In particular, the newly and early arrived immigrants seemed to experience on average different challenges during mathematics testing. The newly arrived students seemed more challenged with terminology but less with the mathematical content while the opposite seemed to hold for the early arrived students. An implication for teaching is that particularly early arrived second language children seem to be in urgent need of support in mathematical concept building from first day of schooling in the new country. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: In press. Paper 3: Manuscript.</p>