On the cover: Exposure to ambient ultrafine particles enhances aortic atherosclerosis in mice. Electron microscopy photographs of a lipid-laden alveolar macrophage harvested from an apoE^–/– mouse exposed to concentrated ambient ultrafine particles (<0.18 μm). Red arrows indicate electron-dense material corresponding to intracellular aggregates of nanoparticles. Notice the subcellular mitochondrial localization encircled by the red dotting. See related article, page 589.