Each cubic millimetre of the grey matter of the neocortex contains 100 000 neurons, over 100 million synapses, and 4 kilometers of axon. Despite this density and inherent complexity, the neocortical circuits of all land mammals, from mouse to man, reflect the laminar distribution of relatively few types of excitatory and inhibitory neurons organized according to common principles of connectivity. These ‘canonical’ circuits are optimized for algorithms that are implemented across the cortical sheet for the full range of demands of behaviour, including perception, cognition and action.