In this paper, Ihave reexamined the previous syntactic theoretical studies on intrasentential code switching with special reference to Spanish and English code switching among hispanic americans in the US. First, Ihave showed Timm(1975)`s five constraints on Spanish/English code switching, which seems to be a descriptive ones. Second, Ihave critically reconsidered some previous syntactic theory-based studies on code switching such as Poplack(1980)`s Equivalence Constraint, Woolford(1983)`s highly Phrase Structureoriented approach, DiSciullo, Muysken and Singh(1986)`s Government Constraint, Mahootian(1993)`s Head Principle, Belazi, Rubin and Toribio(1994)`s Functional Head Constraint, Chan(2008)`s Lexical/ Functional category distinction and finally MacSwan(1999)`s Minimalist Approach to code switching. After the discussion of all of these theoretical approach to code switching, Ihave reexamined Lee(2009a)`s Head Constraint on code switching. In this paper, Ihave reconsidered some unexplained code switched data between Spanish and English, trying to resolve them in terms of syntactic theory. Ihave concluded this paper pointing out that the theory internal research on code switching have actually become one of our mayor linguistic areas and this type of study will contribute to the prosperity of generative grammar. The minimal code switching-specific appartus would be assumed as follows: Nothing constrains code switching apart from the requirements of the mixed grammars.