Cover image: Single-molecule observation of catalytic turnover. The wide-field image was recorded during hydrolysis of fluorescein esters on a catalytic monolayer. Individual turnover events appear as isolated bright spots. Microscopic and spectroscopic studies of enzymes and inorganic agents provide evidence that catalysts, even of uniform composition, are ensembles possessing a range of activities. See the Perspective by Roeffaers et al. on pages 12603–12609. Image courtesy of Hiroshi Uji-i.