Membrane potential is a fundamental biophysical property maintained by every cell on earth. In specialized cells like neurons, rapid changes in membrane potential drives the release of chemical neurotransmitters. Coordinated, rapid changes in neuronal membrane potential across large numbers of interconnected neurons forms the basis for all of human cognition, sensory perception, and memory. Despite the importance of this highly orchestrated and distributed activity, the traditional method for recording membrane potential is through the use of highly invasive, singlecell electrodes that offer only a small glimpse of the total activity within a system. Fluorescent dyes that change their optical properties in response to changes in biological voltage have the potential to provide a powerful complement to traditional, electrode-based methods of inquiry. Voltage-sensitive fluorescent indicators would allow the direct observation of membrane potential changes, significantly expanding our ability to monitor membrane potential dynamics in living systems.