Science and mathematics integration has long been recommended as a way to increase student conceptual understanding of, interest in and motivation to learn both subjects. However, attempts to develop a model to integrate science and mathematics have not resulted in a consensus regarding optimal curricular organisation. This research therefore has designed and developed a curriculum model for assisting teachers to integrate mathematics into science in second-level education in Ireland. This chapter reports on some of the findings from the evaluation of the artefacts of the model, in particular themes relating to teachers’ perceptions regarding disciplinary boundaries of subject communities. Two major themes (disciplinary disconnect and boundary crossing) were identified and are addressed in this chapter. The first concerns the disconnect between science and mathematics in second-level schools, and the second concerns the potential of engagement with the model to support boundary crossing between subjects. The findings suggest that curriculum models need to take account of the subject subculture, school structure and teacher subject identity issues that impact on the curricular choices teachers make.