Institut du Fer a Moulin 1 article published in JoVE Neuroscience Two-photon Imaging of Microglial Processes' Attraction Toward ATP or Serotonin in Acute Brain Slices Fanny Etienne1,2,3, Vincenzo Mastrolia1,2,3, Luc Maroteaux1,2,3, Jean-Antoine Girault1,2,3, Nicolas Gervasi*1,2,3, Anne Roumier*1,2,3 1Institut National de la Santé et de la Recherche Médicale (INSERM), UMR-S 1270 Paris, France, 2Sorbonne Université, Paris, France, 3Institut du Fer à Moulin, Paris, France Microglia, the resident immune cells of the brain, respond quickly with morphological changes to modifications of their environment. This protocol describes how to use two-photon microscopy to study the attraction of microglial processes toward serotonin or ATP in acute brain slices of mice.