This pseudocolor image shows a vertical reconstruction of a stack of two-photon microscopy images of the living mouse retina labeled with a fluorescent dye that fills all extracellular spaces. The photoreceptor layer is at the top and the retinal ganglion cell layer at the bottom. The volume of the extracellular spaces between cells in each retinal layer can be determined by measuring the local brightness of the fluorescent dye. Changes in extracellular volume evoked by light stimulation can be measured by changes in dye brightness. For more information, see the article by Kuo et al. (pages 7785–7794). Cover Image: Eric A. Newman.