Abstract: As a universal phenomenon in human speech, coarticulation started to draw the attention of Western scholars as early as the nineteenth century. While its classification and basic features have been clarified, in-depth studies in recent years have shown coarticulation to be a process more closely related with the phonology-grammatical system of a language than purely phonetic. Consequently, linguists began reflecting on the nature of coarticulation and sought to formalize coarticulatory processes, leading to the development of numerous theories and models. This paper summarizes major theories and models of coarticulation proposed by Western linguists since 1933, when the concept of coarticulation was first introduced. We categorize these theories into three broad groups and briefly introduce the most representative models within each category. The paper then highlights the key areas of dispute between these theories and models, comparing the strengths and weaknesses of the latter two major categories of theories, which are based on distinctive features and articulatory gestures respectively, in addressing these disputes. Finally, the paper points out the directions for the future development of theories and research on coarticulation.