Most languages have numeral noun phrases resulting from combining nouns directly with numerals (e.g., ‘three cows’). In a wide variety of languages, however, usual translations of such phrases have special expressions intervening between nouns and numerals. The special expressions are called numeral classifiers, and languages with a substantial system of (numeral) classifiers are called (numeral) classifier languages. This chapter gives a more precise characterization of classifiers and discusses various classifier systems. In doing so, it elaborates on the classifier systems of Mandarin Chinese, Japanese, and Korean and argues that all these languages have nouns that can directly combine with numerals. The chapter also discusses languages (e.g., Itzaj Maya, Korean) that can have both classifiers and plural markers in the same numeral noun phrases.