Lesson Study (LS) is a teaching improvement and knowledge-building process that has origins in Japanese elementary education. In Japanese LS, teachers work in small teams to plan, teach, observe, analyse and refine individual lessons called research lessons. This study examined a small sample of primary school teachers' perceptions of LS as a professional learning endeavour. The benefits and challenges teachers experienced when attempting to engage in LS was further explored. The study focused on the Fijian primary teaching context, specifically on Year 8 mathematics teachers. Data was collected using a series of class observations and semi-structured interviews in two case-study schools. Analysis of classroom observations and semi-structured interviews confirms that LS provided a useful mode for teachers to talk about their mathematics lessons and open them for scrutinisation by their teaching colleagues. The findings suggest that all the teachers in the two schools found that LS is a powerful learning platform to improve teachers' mathematical knowledge and pedagogical skills. These findings have important implications for the implementation of effective professional learning amongst practising primary school teachers.