Pyramidal cells in the cerebral cortex exhibit large and elaborate dendrites, yet whether they are simply decorative cables or serve a function was, for many years, unclear. Historically, dendrites were believed to convey the information received from their thousands of synaptic inputs in a passive manner, and neurons were thought to linearly sum the weighted synaptic inputs at the cell body to decide whether to fire an action potential. Early electrophysiological studies showed that, whereas proximal synapses (those located close to the cell body) have a relatively large effect on a neuron’s membrane potential, the effects of most (more distal) synapses are markedly attenuated by the passive cable properties of dendrites. This prompted researchers to question why neurons require so many distal, weak synapses, most of which have minimal influence on the generation of action potentials, and what their functional importance might be.