Purpose: This paper focuses on prospective teachers' (PTs) participation in a lesson study (LS) that prompted them to research their own practice. We seek to describe the dimensions of PTs' knowledge of student learning developed during the process and the LS features fostering it. Design/methodology/approach: The participants were two PTs, a teacher educator, a cooperating teacher and a researcher. The LS was integrated into a Portuguese initial elementary teacher education program. Following a qualitative approach, we used participant observation. Findings The PTs developed their knowledge of students' learning of the concept of area in four dimensions: theories; students' interests and expectations; ways students interact with the content and students' strengths and weaknesses in learning the concept. To support this development, the LS design considered follow-up sessions and emphasised collaborative work. Originality/value: This study focuses on PTs researching their practice and disseminating the results, which has been overlooked in previous research of LS with PTs. The results highlight the potential of LS to motivate PTs to research their practice and emphasise the importance of involving them in disseminating LS results.