The unique ability of a Josephson junction to control the flow of magnetic flux quanta leads to a perfect relationship between frequency and voltage. Over the last 30 years, metrology laboratories have used this effect to greatly improve the accuracy of dc voltage standards. More recent research is focused on combining the ideas of digital signal processing with quantum voltage pulses to achieve similar gains in ac voltage metrology. The integrated circuits that implement these ideas are the only complex superconducting electronic devices that have found wide commercial application.