Voltage-activated cation channels have pores that are selective for K(+), Na(+) or Ca(2+). Neurons use these channels to generate and propagate action potentials, release neurotransmitters at synaptic terminals and integrate incoming signals in dendrites. Recent X-ray and electron microscopy studies of an archaebacterial voltage-activated K(+) (Kv) channel have provided the first atomic resolution images of the voltage-sensing domains in Kv channels. Although these structures are consistent with previous biophysical analyses of eukaryotic channels, they also contain surprises, which have provoked new ideas about the structure and movements of these proteins during gating. This review summarizes our current understanding of these intriguing membrane proteins and highlights the open questions.