People with a working knowledge of two or more languages have the capacity to map thoughts onto two or more parallel orthographic or phonological forms. This ability raises an issue central to bilingual lexical processing research: the extent to which the functional architecture associated with processing words in one language subserves that of the other. Rather than examining a wide range of lexical properties, theories of bilingual lexical processing have typically focused on characterizing the process of lexical access (i.e., how semantic information is retrieved via orthographic and phonological representations). In this regard, the research question typically concerns the relationship between the surface form (orthography or phonology) of the words of each language and the mental representations of the same underlying concept. In this chapter, we consider a number of theories of bilingual lexical access and attempt to demonstrate how recent research using the masked priming procedure provides some support but also a challenge to these. We begin by surveying two prominent theoretical accounts and then describe how the masked priming procedure has been used to investigate these.