Treatment of brain disorders relies on efficient delivery of therapeutics to the brain, which is hindered by the blood–brain
barrier (BBB). The work of Prof. Margareta Hammarlund-Udenaes was instrumental in understanding the principles of
drug delivery to the brain and developing new tools to study it. Here, we show how some of the concepts developed in her
research can be translated to in vivo 2-photon microscopy (2PM) studies of the BBB. We primarily focus on the methods
developed in our laboratory to characterize the paracellular diffusion, adsorptive-mediated transcytosis, and receptor-mediated
transcytosis of drug nanocarriers at the microscale, illustrating how 2PM can deepen our understanding of the mechanisms
of drug delivery to the brain.