Lesson study (LS) is a collaborative practice of inquiry in which teachers design a lesson plan and work to improve it and its execution after observing its instruction. Originating in Japan, LS is recognised in international research as a useful mechanism for teachers' training and professional development. However, research reveals that misconceptions arise when LS is adopted outside of Japan, and different authors have called for further theoretical development to increase comprehension of the process. In response, we analyse three LS' key components (phases, product and teachers' cooperation) from the perspective of the epistemology of complexity, highlighting the role of emergence, the ecology of action, and joint reflection. We suggest that viewing LS through the lens of complexity can allow teachers to gain a deeper understanding of this practice and to apply it more successfully.