The first clear vision of the brain as a network stemmed from the extraordinary work by Ramón y Cajal, the Nobel Laureate (1916) who defined the neuron and conceived of synaptic connections between neurons to form a cellular or microscopic network (see Figure 5.18.1A) on a spatial scale of micrometres to nanometres (i.e. 10−6–10−9 metres). The mammalian brain also has a network organisation at a larger, mesoscopic scale (i.e. 10−4–10−6 m), with cortical areas or subcortical nuclei connecting to each other via bundles or tracts of long-distance axonal projections (see Figure 5.18.1B), and at the macroscopic scale (i.e. centimetres to millimetres, or 10−2–10−4 m), as can be measured using brain MRI (see Figure 5.18.1C). Increasingly detailed maps and technically dazzling images of micro, meso and macro brain network organisation are being produced across a range of species, from the nematode Caenorhabditis elegans, through Drosophila species, to rodents and monkeys. Several books and reviews provide more detail and scope on the flourishing field of connectomics or network neuroscience [1–5]; here we focus on a few high-level principles.