To enact purposeful behaviors, animals acquire and transform sensations into neural representations and memories, and calculate and execute decisions based on recent and past experiences. Our own brains are staggeringly complex, with billions of neurons networked by trillions of synapses. But the building blocks – molecular and cellular structures and interactions – are shared with animal relatives. We study brain and behavior in the roundworm C. elegans and the Drosophila larva. Applying advances in light and electron microscopy, we are able to map, manipulate, and monitor the neural circuits that link brain and behavior in these small creatures.