Scientists will have unprecedented insights into the millisecond-by-millisecond actions of neurons thanks to the development of a tool that allows imaging of individual neurons' electrical activity in live animals. Understanding these dynamics holds important implications for brain research. Electrical signals between neurons are critical to cell-to-cell communication. Neuroscientists have been looking for tools that allow optical imaging of this activity (called membrane voltage dynamics) in awake animals, with little progress. Here, Yiyang Gong et al. dramatically improved upon earlier neuron membrane voltage indicators by fusing a molecule that is highly sensitive to membrane voltage with a fluorescent protein. They used a virus to deliver these indicators to neurons. These molecules could accurately pinpoint the action potential of neurons within less than one millisecond, and nearly eliminate the chances for false readings. In the brains of conscious mice and fruit flies, the researchers successfully recorded action potentials and membrane voltage dynamics. This development offers researchers the long-sought means of seeing brain processing in living animals.