This chapter provides an overview of the research topic, primarily demonstrating that the particle Zhe in Northwestern Chinese dialects exhibits numerous unique grammatical features. Although past literature has noted these phenomena, the explanations provided lack persuasiveness. In fact, the distinctive features of Zhe in Northwestern Chinese are not found in its cognates in Standard Mandarin and other Chinese dialects. In light of this strong regional limitation, I propose a new hypothesis: the various unique features of Zhe in Northwestern Chinese result from grammatical evolution catalyzed by contact between Northwestern Chinese and non-Han languages in Northwest China. This book covers in-depth fieldwork and analysis to verify this hypothesis. This chapter outlines the research questions that will guide the subsequent chapters and provides a brief introduction to several commonly-used function words and grammatical structures in Chinese, thereby facilitating the reader’s comprehension of the book’s content.