Edward Boyden wants to unravel the complex network of the human brain – roughly 100,000 cells per cubic millimeter, connected by a billion synapses, each operating with millisecond precision – to understand the neuronal underpinnings of our thoughts and feelings. Boyden and his team developed expansion microscopy to image structures with nanoscale precision, and map how information flows in the brain. They also created and employ optogenetic tools to control nerve cells’ electrical pulses. The team is also developing novel methods for noninvasive focal stimulation of deep brain neurons, and new reagents for imaging high-speed physiological events in the nervous system. Boyden and his team apply these tools to reveal fundamental mechanisms of brain function, and yield therapeutic strategies for treating neurological and psychiatric disorders.