Mapping neural circuit is one of the central goals in neuroscience. Modern neuroscience has established that connectivity between neurons is key to any brain functions. Since more than a hundred years ago, morphology of individual neurons and abstract wiring diagrams between neurons have been illustrated by tracing small numbers of neurons labeled in one or two colors. Yet complete wiring diagram between all neurons is unknown even for the simplest circuit, primarily due to the difficulties of assigning identities to all neuronal processes in a densely labeled sample. To address this challenge, a transgenic labeling strategy, Brainbow was developed, marking neurons with many different colors. In Brainbow mice, combinatorial expression of fluorescent proteins can endow individual neuron with a random color from a hundred-color pallet. Thus nearby neurons are distinct by their color identities. In parallel, computer software is under development to automatically identify colors and trace neuronal processes. Using these tools, we are able to, for the first time, construct densely labeled neural circuits at the cellular resolution in a mammalian brain.