This chapter discusses a framework for analysing the role of mathematics during physics lessons in upper-secondary school. It takes as a starting point that relations made during physics lessons between Reality, Theoretical models and Mathematics are of the outmost importance. The framework was developed to analyse the communication during physics lessons. It was developed during a pilot study exploring the role of mathematics for physics teaching and learning in upper-secondary school during different kinds of physics lessons (lectures, problem solving and labwork). In the overall project observations have been made in three physics classes (in total 7 lessons) led by one teacher. Here the developed analytical framework is described together with selected results from one class (3 lessons) showing how the framework could be used. This chapter describes and discusses the uses of the framework and shows how results on students and teachers usages of links between the three entities Reality, Theoretical models and Mathematics can be brought to the forefront in an analysis of complex physics teaching situations. And, how the framework can be used to analyse the different organisational forms: lectures, problem solving in groups and labwork.