Much SDT research in the language learning domain focuses on the learning of English as a second or foreign language, in many cases by undergraduate students. By contrast, our work focuses on adolescent learners (aged between 11 and 16) of what are termed Modern Foreign Languages--languages other than English--in schools in England. In this context, school-level policy means that language learning is often optional for students beyond the age of 14, and this optionality makes motivation critical. In this article, we suggest that Self-Determination Theory provides a suitable framework for measuring the motivation of such students and compare it with the well-known Second Language Motivation SelfSystem (L2MSS) developed by Dörnyei. We draw on data from two large-scale studies of school students in England (N[subscript 1] = 666; N[subscript 2] = 1797), the first to use the Self-Regulation Questionnaire (Academic) in this context, and a smaller pilot study (N = 338), similarly the first to use the L2MSS in this context, and argue that SDT provides a more useful lens for studying motivation in English schools. As the L2MSS relies heavily on the learner's Ideal Self, a version of themselves who can communicate competently in the target language, we argue that it is not suitable for use with learners in this context, although there is scope to incorporate some elements of it. SDT's lack of focus on the target language community, which is not always within reach for students, allows it to better access students' motivation for study. We consider the implications for future studies of student motivation in an English context, as well as other Anglophone settings.