Abstract: Fluorescent genetically encoded voltage-indicators report the membrane voltages of targeted cell-types. Recent advances in the development of these indicators have enabled optical voltage-imaging experiments capturing the spiking dynamics of up to 3 neuron classes at once in the brains of behaving mammals. We recently used this capability to uncover a new form of interaction between an existing short-term memory and the formation of a long-term memory. Fluorescent voltage-indicators also have the capability to reveal the brain’s electrical oscillations and waves. However, until recently, voltage-imaging instrumentation lacked the sensitivity to track spontaneous or evoked high-frequency voltage oscillations in neural populations. I will describe optical voltage-sensing technologies that capture neural oscillations up to ~100 Hz. With these techniques, we have uncovered synchronized coupling between electrical oscillations of distinct frequencies in specific neuron-types of the hippocampus and neocortex. We have also imaged sensory-evoked excitatory-inhibitory neural interactions and traveling electrical waves in the visual cortex, and discovered previously unreported forms of traveling voltage waves in the hippocampus. Overall, optical voltage-imaging has widespread applications for probing spiking patterns, oscillations, and neuron-type interactions in healthy and diseased brains.