The dynamic behavior of pure actin in vitro is more complex than that of most simple polymers, due to the energy input from the irreversible nucleotide hydrolysis associated with polymerization. However, the dynamic behavior of actin is vastly more complicated inside cells, where dozens of different types of actin-binding proteins alter every rate constant for actin polymerization and the chemical environment is inhomogeneous both temporally and spatially. Actin dynamics in cells are tightly regulated, so that rapid filament polymerization can occur in response to external signals or at the front of an active lamellipodium, while rapid depolymerization occurs simultaneously elsewhere in the cell. Although more direct observations of actin dynamics in vivo are accumulating, it is not yet clear how to reconcile the behavior of actin in cells with its well-documented in vitro properties.