Cell fates are instructed by signals emitted from specialized cell populations called organizers. The study of epidermal patterning in Drosophila is contributing novel insights concerning the establishment and action of such organizers. Juxtaposed rows of cells express either the wingless or hedgehog signaling molecules and thereby act as organizers of segment pattern. These signals mediate a mutually re-enforcing interaction between the two rows of cells to sustain organizer function. In a distinct and subsequent phase, wingless and hedgehog act to specify the fates of cells.