Dendritic spines undergo several types of transformations, ranging from growth to collapse, and from elongation to shortening, and they experience dynamic morphological activity on a rapid time scale. Changes in spine number and morphology occur under pathological conditions like excitotoxicity, but also during normal central nervous system development, during hormonal fluctuations, and in response to neural activity under physiological circumstances. We briefly review evidence for various types of alterations in spines, and discuss the possible molecular basis for changes in spine stability. Filamentous actin appears to be the most important cytoskeletal component of spines, and a growing list of actin-associated and actin-regulatory proteins has been reported to reside within spines. We conclude that spines contain two distinct pools of actin filaments (one stable, the other unstable) that provide the spine with both a stable core structure and a dynamic, complex shape. Finally, we review the current state of knowledge of actin filament regulation, based on studies in nonneuronal cells.