The aim of this research project is to illuminate interest manifested as student engagement in mathematics in years 6-9. In particular, the studies capture how engagement is recognised by teachers and researchers and what didactical strategies the teachers use to engage students in an introduction to algebra. Also, tasks seen by students as interesting and engaging are presented and analysed. Unlike other studies, student engagement is discussed in light of the Theory of Didactical Situations in Mathematics (TDS). The most important results are insights into the relational constitution of engagement. These insights are visible in the interplay between the student, the teacher, the task and the mathematics. The results show that teachers have an important role in engaging students in mathematics during the didactical situation. Teachers seem to agree on how engagement is indicated in the classroom. The strategies for enhancing engagement provided and discussed by the teachers are all a part of the meso-contract. Further, working with the target knowledge in the foreground can enhance student engagement and thus contribute to the development of an adidactical situation. These empirical findings seem to support the idea that, in order to engage students in mathematics, it is important to design didactical situations and tasks where enhancing engagement is a part of the macro-contract.