The most distinctive characteristics of Mexican American language is in the sustained usage of Spanish and English in a single sentence or discourse: code switching. Here in this paper, I will focus on the language situation of Mexican Americans: the bilingualism of the community that has persisted for more than 150 years. Specifically, I will be concerned entirely with Spanish/English code switching among Mexican American bilinguals. This paper will be composed as follows. In Chapter 2, I will briefly introduce the general aspects of Mexican Spanish in the United States. Chapter 3 deals with forms and meanings of code switching: I will c1early distinguish code switching from the other terms such as borrowing and interference (i.e. Calque). In this chapter, I will also consider some functional types of code switching found in Mexican American speech. In chapter 4, I will describe a general and simple syntactic constraint on code switching in the research literature: code switching is possible on1y when it does not violate a syntactic rule of either language. In chapter 5, the conclusion of this paper will be presented.