In this article we explore the ways critical thinking is conceived by a group of Singaporean social studies teachers, what they see as its purposes as well as perceived constraints to critical thinking and teaching critical thinking in Singapore's schools. Using a case study research design and constant comparative method we analysed data from teachers' discussion board entries, observation notes and lesson artefacts from a Master's class. Findings revealed three key tensions involving teaching critical thinking in an exam culture, uncertainty about what constitutes the "out-of-bound", and the issue of professional identity. Each of these tensions intersected and interacted in dynamic ways for teachers and shaped the way they understand and practise critical thinking. (Contains 1 table.)