This study serves as a response to a development need where difficulties regarding students' ability to solve mathematical problems were observed in two schools. The aim of this study has been to investigate the effects of using cooperative learning when teaching mathematics for students’ mathematics development. Specifically, it examines students’ conceptual and procedural knowledge in problem solving with a focus on proportionality. To study this, the following three questions were posed: 

What differences are identified in the students' post-tests regarding their procedural and conceptual knowledge in problem solving with a focus on proportionality? What basic cooperative principles are made visible in the student groups during the 

problem-solving lessons? How can cooperative learning contribute to students' mathematics development with a 

focus on conceptual and procedural knowledge in the field of problem solving? 

To answer these questions, qualitative text analyses have been carried out with the help of an analysis tool based on theories of conceptual and procedural knowledge. The texts have consisted of eight selected students' pre- and post-tests consisting of problem-solving tasks with a focus on proportionality. Between the pre- and post-tests, a total of four cooperative lessons have been completed and observed based on a structured observation schedule founded on the five basic principles of cooperative learning. The results of the study show that there has been a development in seven out of eight students regarding their conceptual and procedural knowledge. During the cooperative lessons, all five basic principles for cooperative learning could be observed, although to varying degrees in the different working groups. Based on this, it can be interpreted as reasonable that there are certain correlations between specific students' mathematics development and cooperative learning. Therefor it is plausible that cooperative learning can be assumed to have a positive effect on students’ mathematics development. However, it is important to keep in mind that cooperative learning does not automatically contribute to the mathematical development of students, nor is it a form of work that suits all students.