This paper is roughly a literature review. Metaphor is pervasive in language, and has been studied from many approaches, including rhetoric, cognitive linguistics, pragmatics, natural language processing, and experimental psychology. The research topics include rhetoric effects, cognitive mechanisms, interpretation mechanisms, recognition and semantic processing. The paper firstly provides detailed reviews of metonymy studies from the perspectives mentioned above, and then points out the strengths and weaknesses of each approach as well as the gaps future research needs to fill in, that is, the lack of an integrated approach. And it also makes clear that issues like interpretation mechanism of metonymy deserve further investigation.