Over the years, there has been a wealth of psycholinguistic research investigating the lexicon, or an individual’s mental dictionary. Some researchers study the mental lexicon in monolinguals, or individuals who speak only one language. Other psycholinguists study bilinguals, or individuals who speak more than one language. Studying bilinguals allows researchers to ask questions about how words from two languages are organized and retrieved from the lexicon. But within this area of research, the term bilingual is sometimes applied to individuals who are highly proficient in two languages that they learned simultaneously from birth, as well as to individuals who are learning a language in the university setting. It is obvious that these are very different types of bilinguals. In fact, the terms second language (L2) learner, beginning bilingual, and advanced bilingual better describe those individuals learning a language later in life in an academic setting. Yet, on some level, wherever the individual falls on the language skill spectrum, that individual is no longer a monolingual. Thus, research investigating the bilingual lexicon and bilingual memory is concerned with individuals who have words from more than one language in their minds, from beginning stages of language learning through proficient, highly-skilled use. Scholars interested in second language acquisition often focus on developmental aspects of the bilingual lexicon investigating beginning bilinguals (i.e., L2 learners). However, they also focus on individuals with more advanced language skills as well, as a point of comparison in terms of lexical development. As a result, throughout this chapter, various types of individuals with differing language abilities will be discussed. The terms L2 learner and beginning bilingual will often be used interchangeably, whereas the term highly-proficient bilingual will be used for those with more advanced skill in a second language.