A modularly organized brain mechanism, specialized for number appreciation and processing, seems to have been developed by evolution. The process of capturing numerosity and accessing the corresponding mental representation is performed via three basic tasks: subitizing, counting, and estimation. Number representation is believed to be analogical. Calculation relies on a general ability, conceptual knowledge, on specific abilities for dealing with procedures and on a specialized memory system for rote arithmetical knowledge. Neuropsychological investigations have revealed several independent subsystems in both number processing and calculation, and partly clarified their complex relation with processing of language.