The United States is formally represented around the world by approximately 14,000 Foreign Service officers and other personnel in the U.S. Department of State. Roughly one-third of them are required to be proficient in the local languages of the countries to which they are posted. To achieve this language proficiency for its staff, the State Department's Foreign Service Institute (FSI) provides intensive language instruction and assesses the proficiency of personnel before they are posted to a foreign country. The requirement for language proficiency is established in law and is incorporated in personnel decisions related to job placement, promotion, retention, and pay. "A Principled Approach to Language Assessment: Considerations for the U.S. Foreign Service Institute" evaluates the different approaches that exist to assess foreign language proficiency that FSI could potentially use. This report considers the key assessment approaches in the research literature that are appropriate for language testing, including, but not limited to, assessments that use task-based or performance-based approaches, adaptive online test administration, and portfolios.