The brain is a challenging organ to understand because its functions arise from electrical signals transiently passing through billions of neurons via trillions of synaptic connections, whose strengths change over time. Methods for visualizing electrical activity, biochemical changes, and structure in genetically defined neuronal subpopulations, and for manipulating the electrical and biochemical state of individual neurons, will be critical for unraveling this complexity. I will present new genetically encoded optical tools for sensing voltage with millisecond resolution and for visualizing late stages of synaptic plasticity occurring over hours. I will also present an uniquely generalizable method for reversibly controlling biochemical activities in the cell with light. Unexpectedly, all these methods use the same domain for either sensing or control, the fluorescent protein domain.