The Wiskott–Aldrich syndrome protein (WASP) is an important regulator of the actin cytoskeleton that is required for many haematopoietic and immune cell functions, including effective migration, phagocytosis and immune synapse formation. Loss of WASP activity leads to Wiskott–Aldrich syndrome, an X-linked disease that is associated with defects in a broad range of cellular processes, resulting in complex immunodeficiency, autoimmunity and microthrombocytopenia. Intriguingly, gain of function mutations cause a separate disease that is mainly characterized by neutropenia. Here, we describe recent insights into the cellular mechanisms of these two related, but distinct, human diseases and discuss their wider implications for haematopoiesis, immune function and autoimmunity.