This study explores the conceptual semantics of numbers and counting, using the natural semantic metalanguage (NSM) technique of semantic analysis (Wierzbicka 1996; Goddard & Wierzbicka (eds.) 2002). It first argues that the concept of a number in one of its senses (number1, roughly, "number word") and the meanings of low number words, such as one, two, and three, can be explicated directly in terms of semantic primes, without reference to any counting procedures or practices. It then argues, however, that the larger numbers, and the productivity of the number sequence, depend on the concept and practice of counting, in the intransitive sense of the verb. Both the intransitive and transitive senses of counting are explicated, and the semantic relationship between them is clarified. Finally, the study moves to the semantics of abstract numbers (number2), roughly, numbers as represented by numerals, e.g. 5, 15, 27, 36, as opposed to number words. Though some reference is made to cross-linguistic data and cultural variation, the treatment is focused primarily on English.