Key steps in regulating actin dynamics are the de novo nucleation and elongation of actin filaments, which can be catalysed by a limited number of proteins and protein complexes. Among these, Arp2/3 complex and formins are the best studied. Arp2/3-complex activity is controlled through signalling-dependent association with nucleation-promoting factors, such as the WASP/WAVE family proteins. A common theme for these molecules, which is well established for WAVEs but is only just beginning to emerge for WASPs, is that they act as coincident detectors of a variety of signalling pathways through the formation of large multi-molecular complexes.