For a few years now, I’ve taught a course at MIT called “Principles of Neuroengineering.” The idea of the class is to get students thinking about how to create neurotechnology innovations—new inventions that can solve outstanding scientific questions or address unmet clinical needs. Designing neurotechnologies is difficult because of the complex properties of the brain: its inaccessibility, heterogeneity, fragility, anatomical richness, and high speed of operation. To illustrate the process, I decided to write a case study about the birth and development of an innovation with which I have been intimately involved: optogenetics—a toolset of genetically encoded molecules that, when targeted to specific neurons in the brain, allow the activity of those neurons to be driven or silenced by light.