6 ASSESSMENT OF LANGUAGE PROFICIENCY AND DOMINANCE IN MONOLINGUALS AND BILINGUALS Elizabeth D. Peña, Lisa M. Bedore, and Julio Torres Introduction In global and international contexts, the use of two or more languages by single individuals is not uncommon. Bilingualism, however, is not well understood, and over the last 25 years it has been the subject of increasing interest.A fundamental question in the study of bilingualism is how to measure it. There are numerous ways of quantifying and qualifying bilingual exposure, experience, use, profciency, and ability.The ways that researchers do this shape our understanding of bilingual phenomena. How we defne and quantify levels of bilingualism has practical and important applications in education, policy, and the marketplace. Bilingualism is of interest to psychologists, linguists, and educators.The study of bilingualism and bilingual individuals informs research about language pro- cessing, organization, and language access. To better understand and interpret research results, it is important to have full descriptions of participants’ language fuency. In applied contexts, it is also important to know to what extent a person is bilingual. In school settings, decisions about programming and language of instruction require data-based assessments. In the workplace, it may be important to know if someone can communicate in another language. Measuring Bilingualism Aspects of bilingualism that are important to measure include language dominance, profciency, ability, use, exposure, and experience. Some of these terms are at times used interchangeably.Thus, it is critical to defne what each of these means. Language dominance typically refers to relative profciency or ability between two languages. The dominant language is usually the one in which a person