This project offers an analysis of complexity, accuracy, and fluency (CAF), a triad that represents important constructs in the study of Second Language Acquisition (SLA). The overarching goal of this project is to analyze the interlanguage of English as a Foreign Language (EFL) learners attending German schools based on empirical data. Therefore, we will compile and annotate a corpus of spoken and written production data from learners in early to intermediate stages (grades 5–12 in the German system). In addition, we will investigate CAF in this group, explore the interdependence of these variables and attempt to explain variation with regard to contextual and learner factors (e.g. motivation, socio-economic status). The quasi-longitudinal approach applied will serve to model the trajectory of EFL acquisition in the German context. We will take a usage-based perspective with a focus on the CAF triad and potential interactions between its individual components. The project will serve to overcome several of the longstanding restrictions of LCR (such as a bias towards studying learners at advanced proficiency levels) and will contribute to a better understanding of processes of SLA more generally, and of the analysis of the CAF triad specifically.