Actin filaments can assemble at the barbed end and disassemble simultaneously at the pointed end. A higher monomer concentration is required to balance the association of actin monomers and the dissociation of filament subunits at the pointed end than at the barbed end. This treadmilling reaction or disparity of the apparent affinity of the two ends for monomers is caused by a continuous hydrolysis of adenosine triphosphate occurring during the association of a monomer with a filament end. In this article, in vitro investigations on treadmilling are reviewed and emerging physiological implications are discussed.