Designing tasks to foster students' learning is challenging for teachers. In this article, we aim to understand how designing, enacting, and reflecting on tasks promotes the development of in-service and prospective teachers' didactical knowledge during lesson study. Data was collected through participant observation with audio recordings and document collection. Data analysis was based on a category system for task design and students' work and a teachers' didactical knowledge framework. The results suggest that, despite their different teaching experiences, there are aspects that both in-service and prospective teachers attended to when designing tasks. However, in-service teachers had clearer aims when designing tasks and changed them, after observing and reflecting on students' work. Moreover, proposing tasks and reflecting on students' work allowed both groups to rethink their practice to foster students' learning, highlighting the potential of lesson study to deepen their knowledge.