The dendritic spine may be considered a fusion of a specialized actin-based structure akin to filopodia and lamellopodia, with an excitatory postsynaptic density containing glutamate receptors and signal-transducing machinery. This specialized neuronal microdomain is the site of the majority of excitatory synaptic contacts in the mammalian brain. Regulation of spine morphology, composition, and stability are likely to contribute to long-lasting changes in synaptic efficacy. Thus, understanding the function and regulation of dendritic spines is a fundamental problem ranging from molecular through behavioral neurobiology. A complete understanding of dendritic spines will require a knowledge of all the molecular components and how these components interact. Here we wish to accomplish two goals: to catalog many of the known components of hippocampal dendritic spines and suggest how these may contribute to spine function; and to compare dendritic spines with other actin-based structures, namely lamellopodia, filopodia, microvilli, and stereocilia, to gain some insight into possible common vs. specialized mechanisms of regulation of the shape, motility, and longevity of these actin-based structures.