The Arp2/3 complex is necessary for nucleating the formation of branched networks of actin filaments at the cell cortex, and an increasing number of proteins able to activate the Arp2/3 complex have been described. The Wiskott-Aldrich syndrome protein (WASP) family and cortactin comprise the large majority of the known activators. WASPs bind to Arp2/3 via an acidic (A) domain, and a WH2 domain appears to bring an actin monomer to Arp2/3, promoting the nucleation of the new filament. Cortactin also binds the Arp2/3 complex via an A domain; however, it also binds to actin filaments, which helps activate the Arp2/3 complex and stabilise the newly created branches between the filaments.