As part of the trend toward internationalization of higher education, governments and universities have introduced policies to encourage the expansion of English-medium instruction (EMI). However, top-down policies do not necessarily translate to teaching and learning practices. This article provides a case study examining the implementation of undergraduate EMI engineering programs at a state university in Turkey to explore the gaps that exist between national- and institutional-level EMI policies and classroom-level practices. Data were collected through policy documents, classroom observations, semi-structured interviews with teachers, and focus group discussions with students. The findings suggest that the implementation of EMI varies across classrooms, even within the same university department. Despite policies that envision one-language-at-a-time instruction, the EMI lecturers in this study varied in terms of language preference and teaching practice in their EMI lectures. Implications are discussed with respect to policy planning, teacher training, and the expansion of EMI across university contexts.