Cepheids help to map the Galaxy Cepheid variable stars pulsate, which allows their distances to be determined from the periodic variations in brightness. Skowron et al. constructed a catalog of thousands of Cepheids covering a large fraction of the Milky Way. They combined optical and infrared data to determine the stars' pulsation periods and mapped the distribution of Cepheids and the associated young stellar populations across the Galaxy. Their three-dimensional map demonstrates the warping of the Milky Way's disc. A simple model of star formation in the spiral arms reproduced the positions and ages of the Cepheid population. Science , this issue p. 478