Dr. Tri Phan is a Senior Research Fellow at the Garvan Institute of Medical Research and a Conjoint Professor at the University of New South Wales. He runs the Intravital Microscopy Lab, which aims to understand the checks and balances that control B cell responses. His team uses two-photon microscopy to image cells in live animals, including time-lapses of over 24 hours. He talks about the physics behind this type of microscopy and how it can image deep within tissues without destroying them. We also discuss his lab’s recent Cell paper on osteomorphs, a newly discovered cell type involved in osteoclast recycling in bone, and their work on memory B cells in lymph nodes.