Cofilin is a small phosphoinositide sensitive, actin binding protein capable of depolymerizing actin filaments in vitro. Under certain conditions, it fragments the filaments and accelerates actin subunit dissociation from their ‘pointed’ (minus) ends. Cofilin binds stoichiometrically to monomeric G-actin and to actin protomers in filaments in an apparent pH-dependent, Ca2+- independent manner. Cofilin intercalates between longitudinally associated actin monomers within the filament and distorts its helical twist. Cofilin is ubiquitous in tissues of eukaryotes and is especially abundant in neuronal tissues. It is essential for viability and is important for many cellular processes involving actin remodeling such as motility at the leading edge of cells, polarized cell growth, endocytosis, phagocytosis, cellular activation, cytokinesis, and pathogen intracellular motility.