Beginning ten years ago, Karl Deisseroth and several of his students developed the most disruptive technology neuroscience had seen in a long time. Optogenetics is a method to control single kinds of brain cells with a light-triggered switch, allowing scientists to turn on and off specific cells and connections in the brain and to map neural circuits with extremely high precision. The technique was freely distributed to thousands of laboratories and launched a new era in neurobiology research and therapy. Deisseroth, who is also a practicing psychiatrist, applies his methods in the clinic to create better treatments for mental disorders. In the near future, new devices and treatments based on insights from optogenetics might be used to treat illnesses such as Parkinson’s disease, anxiety, or schizophrenia. In 2013, Karl Deisseroth presented yet another game-changing technique, which provides a way to make intact organs such as the mammalian brain transparent to light, allowing detailed insights into its structures and pathways. Since then, laboratories around the world have begun using the technique, called CLARITY, to gain three-dimensional views of complex neural networks and map brain functions. At Falling Walls, Karl Deisseroth explains how optogenetics and CLARITY can shed light on, and ultimately help address, mental illnesses, which remain some of the least understood phenomena in medicine.