What happens in the brain when we see, hear, think and remember? To be able to answer questions like this, neuroscientists need information about how the millions of neurons in the brain are connected to each other. Scientists have taken a crucial step towards obtaining a complete circuit diagram of the brain of the mouse, a key model organism for the neurosciences. Researchers have developed a method for preparing the whole mouse brain for a special microscopy process. With this, the resolution at which the brain tissue can be examined is so high that the fine extensions of almost every single neuron are visible.