Healthy human brains come equipped with several circuits that contribute to number processing. Nature and nurture interact to produce a unique combination of core skills and more sophisticated abilities, by building on a handful of auxiliary routes (e.g., verbal language, body knowledge and visuospatial attention). Transcranial magnetic stimulation (TMS) studies on number processing will be here succinctly reviewed, in light of their most stimulating and challenging contributions. New research directions will be pointed out, that might enhance their theoretical impact.