A fundamental problem in neuroscience is understanding how a neuron transduces synaptic input into action potentials. The dendrites form the substrate for consolidating thousands of synaptic inputs and are the first stage for signal processing in the neuron. Traditionally, dendrites are viewed as passive structures whose main function is to funnel synaptic input into the soma. However, dendrites contain a wide variety of voltage- and time-dependent ion channels. When activated, the currents through these channels can alter the amplitude and time course of the synaptic input and under certain conditions even evoke all-or-none regenerative potentials. The synaptic input that ultimately reaches the soma is likely to be a highly transformed version of the original signal. Thus, a key step in understanding the relationship between synaptic input and neuronal firing is to elucidate the signal processing that occurs in the dendrites.