The Wiskott-Aldrich syndrome protein (WASp) is a member of a unique family whose members share similar domain structures and are responsible for the transduction of signals from the cell membrane to the actin cytoskeleton. For WASp, the interactions with Rho family GTPases and the cytoskeletal organising complex Arp2/3 are critical to these functions, which when disturbed translate into abnormalities of haematopoietic cell signaling, polarisation, migration and phagocytosis. This review discusses the evidence for regulation of highly dynamic cytoskeletal structures by WASp and the consequences of disturbed function on some of these processes.