The nearly 2,000 glomeruli that cover the surface of the olfactory bulb are so distinctive that they were noted specifically in the earliest of Cajal's catalogues. They have variously been considered a functional unit, an organizational unit and a crucial component of the olfactory coding circuit. Despite their central position in olfactory processing, the development of the glomeruli has only recently begun to be investigated with new and powerful genetic tools. Some unexpected findings have been made that may lead to a new understanding of the processes involved in wiring sensory regions of the brain. It may no longer be sufficient to simply invoke genes, spikes and their interplay in the construction of brain circuits. The story of 'how the olfactory bulb got its glomeruli' may be more complex, and more revealing, than has been supposed.