Many people around the world find it necessary, or useful, to have some amount of proficiency in more than one language. This proficiency is refered to as multilingualism. The proficiency may only be the ability to use or understand a few words of another language, or it may include the ability to read, write, and use the language with eloquence. Multilingualism is a proficiency acquired by an individual, but there tends to be patterns of multilingual proficiency throughout a community. Similar people have similar needs for multilingual proficiency.

The study of multilingualism is important for determining the language repertoire of a speech community and also to assess if people may prefer literacy in a language other than their heritage language. Multilingualism may also be an indication of the communities shift away from the heritage language to another language.

SIL language assessment specialists use quantitative and qualitative methods to study multilingualism. Two methods used for assessing multilingualism are the Reported Proficiency Evaluation (RPE) and the Sentence Repetition Test (SRT). The RPE is an indirect method that involves research participants saying what they “can do” in a second language in specific social situations. The SRT is a test for directly observing actual speaking proficiency. The RPE and SRT are calibrated to proficiency levels of the Interagency Language Roundtable (ILR) six-level scale. In addition, several qualitative methods are used, such as observation, interviewing, and questionnaires.

Along with multilingualism SIL language assessment specialists also study language vitality, language use patterns, and language attitudes.