Understanding what causes numerical difficulties/disorders (henceforth, NDD) is crucial for the prevention of NDD at earlier stages and for a reliable diagnosis of groups with a high risk factor. Such an understanding requires the integration of scientific findings from different fields investigating at the molecular level (genetics, receptors) or the behaviour-brain relations level. Currently knowledge from most of these fields is rather limited or absent. The main focus of this Encyclopedia entry will therefore be the level of the neurocognitive mechanism(s) that subserve NDD.