WASP/WAVE family proteins are important regulators of the Arp2/3 complex, which causes exponential growth of actin filaments. WASP/WAVE proteins mediate actin polymerization for both cellular protrusions, such as filopodia and lamellipodia, and invaginations, such as coated pits for endocytosis. However, it had been unclear how the direction of actin polymerization for these topologically different structures is precisely regulated. Recently, the BAR domain superfamily members, which contain membrane-deforming or membrane-adaptor domains, were found to interact with the WASP/WAVE family proteins. These membrane-deforming or membrane-adaptor domains contain BAR, EFC/F-BAR, and IMD/I-BAR domains, which induce membrane invaginations or membrane protrusions. Due to the various geometries of the membranes bound by the BAR domain superfamily members, these proteins could connect specific membrane structures to actin filaments, mediated by the WASP/WAVE family proteins and the Arp2/3 complex.