In 2004, a report from OECD concluded that Norwegian schools lacked information about the progress of the students, and furthermore did not have sufficient tools for identifying students who needed extra support. After this report, assess meant for learning became a priority in Norwegian schools, and there has been spent much time and resources on developing tools that can be used by teachers during the assessment. In addition, there has been directed a greater focus on how to monitor students in their learning process so that they can further develop their skills throughout primary school. Diagnostic assessment is the topic of this thesis. The purpose is to gain an understanding on how teachers understand diagnostic assessment and the experiences they have linked to this concept. This has led me to the following research question: What kind of experiences do primary teachers have with diagnostic assessment in mathematics? My secondary problem is: How is the work of diagnostic assessment in mathematics conducted at two different schools? The thesis is based on the accounts of teachers' experiences with diagnostic assessment in mathematics. Other key sources are previous research and national policy documents.The thesis is conducted as a social science survey. Qualitative interviews and studies of the theory have been the basis for my information retrieval. The selection consists of two focus groups with a total of nine participants who teach mathematics in primary or lower secondary stages. The survey takes a look at the experiences they have with diagnostic assessment in mathematics and how they use diagnostic assessment tools in their teaching. The thesis explains the term diagnostic assessment within the formative assessment perspective and why diagnostic assessment plays a key role in the school. The thesis will also address the mapping tool Alle Teller. Findings of the study show that teachers have fragmented knowledge of diagnostic assessment in mathematics and how they can use this as a means of formative assessment. The full potential of diagnostic assessment tools i s in this way not used.