Through close analysis of primary source textual documents produced by the Foreign Service Institute (FSI) between 1947 and 1968, this unique text reveals the undocumented influence of the FSI on K-12 language instruction and assessment in the United States. By investigating the historical development of the FSI and its attitudes and practices around language learning and bilingualism, this text provides in-depth insight into the changing value of bilingualism in the US, and highlights how the FSI’s practices around language instruction and assessment continue to influence language instruction in American public schools. By mapping the development and integration of language proficiency assessments which strongly resemble those used by the FSI, historical analysis uncovers key political and economic motivations for increased promotion of language instruction in the US education system. Providing insights into issues of language instruction and assessment in public education that persist today, this book will be particularly useful to researchers and students interested in how policy formation has shaped language instruction and assessment in US public schools.