This book indicates that the nature and function of de is one of the core issues in the study of Chinese grammar. There are many related studies, which are not only a long-term controversial and difficult problem, but also a theoretical issue that constantly invites the renewal of research methods. This book mainly focuses on the following aspects, summarizes the achievements of the previous studies, and expounds the possible research directions in the future: 1. How many morphemes can de be analyzed into? 2. Is the basic function of the attributive containing the word de qualitative or descriptive? 3. What determines the presence and distribution of de? What is the Reference-Object Construction? 4. Can the theory of endocentric construction be used to analyze de? 5. Is de a nominalizer? What is the relationship between the self-designation and the transferred designation of de? How to treat the construction "N de V"? 6. Is de a tense-aspect auxiliary in the event sentences "V de O" and "VO de"? What is the function of the sentence final de? Is de a modal particle? 7. The issues of pseudo-attributive vs. quasi-attributive in understanding de.