Time for an #introduction. Nervous systems — yours, mine, those of mice, fish, and insects and worms — are made up of populations of different kinds of neurons that communicate with one another. Current electron microscopy generates millimeter-scale volumes with the morphology and synaptic connectivity of up to 10^5 neurons. As a scientist at the Allen Institute, I am mapping out the cell types and connectivity rules found in cortex and building computational tools to help others do the same.