Since early anatomical descriptions, the existence of dendritic spines has stimulated intense curiosity and speculation about their regulation and function. Research over the past three decades has described an impressive mutability in dendritic-spine number and morphology under a variety of physiological circumstances. Current evidence favors a proposed model in which two pools of actin filaments, one stable and the other dynamic, support both persistent spine structure and rapid spine motility. Potential functions of spine motility and dynamic actin include regulated protein scaffolding, retrograde signaling and synapse stabilization.