Big Ideas in school mathematics can be seen as overarching concepts that occur in various mathematical topics in a syllabus. Although there has been much interest recently in the understanding of Big Ideas, there is little research done in the assessment of Big Ideas thinking. In this paper, we discuss our research on Big Ideas in School Mathematics. The study consists of three sub-studies: the first sub-study on developing an instrument to measure Big Ideas; two sub-studies on measuring students' and teachers' Big Ideas at test-points before and after a professional development on Big Ideas for primary and secondary school teachers and students.