The crawling movement of a cell occurs by protrusion of its leading edge coordinated with translocation of its cell body and depends upon molecular machinery able to respond to signals from the environment. Protrusion is driven by actin polymerization but t wo alternate forms of the actin machinery co-exist at the leading edge of most motile cells: lamellipodia which seem designed for persistent protrusion over a surface, and filopodia which appear to perform sensory and exploratory functions to steer cells depending on cues from the environment. However, a major gap in our understanding has been to connect the signalling pathway to the motility machinery itself.   Recent correlative light and electron microscopic studies have revealed new details of the supramolecular organization of the lamellipodium and filopodium and provide models for understanding their self-organization in cells.