Most excitatory inputs in the CNS contact dendritic spines, avoiding dendritic shafts, so spines must play a key role for neurons. Recent data suggest that, in addition to enhancing connectivity and isolating synaptic biochemistry, spines can behave as electrical compartments independent from their parent dendrites. It is becoming clear that, although spines experience voltages similar to those of dendrites during action potentials (APs), spines must sustain higher depolarizations than do dendritic shafts during excitatory postsynaptic potentials (EPSPs). Synaptic potentials are likely amplified at the spine head and then reduced as they invade the dendrite through the spine neck. These electrical changes, probably due to a combination of passive and active mechanisms, may prevent the saturation of dendrites by the joint activation of many inputs, influence dendritic integration, and contribute to rapid synaptic plasticity. The electrical properties of spines could enable neural circuits to harness a high connectivity, implementing a "synaptic democracy," where each input can be individually integrated, tallied, and modified in order to generate emergent functional states.