Members of the Wiskott-Aldrich Syndrome Protein (WASP) family control cytoskeletal dynamics by promoting actin filament nucleation by the Arp2/3 complex. The WASP relative, WAVE, regulates lamellipodia formation within a 400 kDa, hetero-pentameric WAVE Regulatory Complex (WRC). The WRC is inactive toward the Arp2/3 complex, but can be stimulated by the RacGTPase, kinases and phosphatidylinositols. We report the 2.3 Å crystal structure of the WRC and complementary mechanistic analyses. The structure shows that the activity-bearing VCA motif of WAVE is sequestered by a combination of intramolecular and intermolecular contacts within the WRC. Rac and kinases appear to destabilize a WRC element that is necessary for VCA sequestration, suggesting how these signals stimulate WRC activity toward the Arp2/3 complex. Spatial proximity of the Rac binding site and a large basic surface of the WRC suggests how the GTPase and phospholipids could cooperatively recruit the complex to membranes.