Polymerization of actin occurs primarily at or in close proximity to the plasma membrane. As a result, a region with a high density of actin filament is usually present at the cell cortex or cell periphery. Polymerization occurs at the plus-end of the actin filament. This is a reversible process with the monomers associating and dissociating from the ends of the filaments. Actin polymerization occurs over three phases – a nucleation phase, which involves the formation of a stable actin nucleus, an elongation phase, in which monomers are rapidly added to the filament at the positive end, and a steady state phase, during which the filament dynamics enter a state of equilibrium.