How are neurons connected? What are the fundamental principles of wiring in the brain, i.e. what is the connection code? What are the implications for computation and function? Just as the gene map carries the rules for the structural design of any given organism, it is thought that the map of all circuits in the brain will help unravel its design principles and reveal much about its functioning. A realm of novel techniques has recently emerged allowing the dissection of neural circuits at various levels to ultimately obtain the so-called connectome describing all connections in a given brain area. However, as opposed to the genetic system in which the underlying code is well known since the pioneering work on the structure of DNA, the corresponding connection code remains a mystery. More than a hundred years ago, however, Ramón y Cajal has suggested to interpret construction plans of the brain by observing the morphology of individual neurons. We are now responding to the challenge laid down by Cajal by developing computational tools and mathematical laws to describe this link between structure and function. Morphology is key to understanding both circuits and computation since it reflects the constraints given by both.

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