This book is the first theoretical study of sound-symbolic expressions in Japanese commonly known as mimetic words. It identifies stringent linguistic constraints on these expressions and demonstrates that they form an intricate linguistic system rather than a collection of ad hoc expressions. It then carefully identifies the sound-symbolic meanings of sound units so as to make the elusive meaning of each sound-symbolic expression fully comprehensible. In addition, this book describes a number of interesting facts about the history of the Japanese language which mimetic words reveal.