This book is about the numeral classifier system and the acquisition of Japanese classifiers by Japanese children. It consists of three parts. First, it provides a general typological characterization of numeral classifier phrases and discusses the semantic properties of numeral classifiers based on an analysis of four languages from four different language families. Second, it extensively reviews classifier acquisition research from eight different languages. Third, it examines the acquisition of numeral classifiers by Japanese preschool children. The importance of the study is that it reveals that young children have a much greater sensitivity to the conceptual underpinnings of the numeral classifier system than was previously considered to be the case.