This qualitative study investigates how the multimodal nature of the lab book facilitates knowledge-making and identity construction in an academic medical physics community located in a Canadian university. The study employs the theoretical framework of Writing, Activity, and Genre Research (WAGR) in combination with the theoretical notions of communities of practice (CoP) and multimodality. The data collected include observational field notes, multimodal lab books collected from, and semi-structured interviews conducted with, five participants in the academic medical physics community. Findings of the study indicate that the lab book, particularly its multimodal features, serves as an important knowledge-making artefact within the medical physics community, providing a means of reifying experimental results and a space for community members to participate and negotiate their professional identities. Implications for further research of the nature of knowledge in medical physics, as well as implications for multimodal genre analysis, are discussed.