This book is based on the author’s PhD thesis. Considering the phenomenon of emphasizing “words” but neglecting “idioms” in lexicography and Chinese information processing, this book reflects upon the research of phrases and idioms, and puts forward the concept of Idiom Unit (IU). The texts from People’s Daily (2001~2003) with a total of about 80 million characters are analyzed in this research. Under the guidance of Dynamic Language Knowledge Renewing Theory and on the basis of Degree of Circulation Theory, the annual average setting value of degree of circulation (0.5) is adopted as the main criterion; the primary selection of IU by combining rules and statistics is conducted, and the quantitative and qualitative analyses on the noise environment of some selected IU are carried out. This book focuses on studying the selected 3-character and 4-character strings, the popular words and phrases, lettered words and phrases, interjections and abbreviations.