The present study investigates the impact of language learning strategies (LLS) and successful second language acquisition/learning of year 9 grammar school students in regular English as a foreign language classes (EFL) as well as of students enrolled in Content and Language Integrated Learning (CLIL) streams. CLIL poses higher language related affordances for students within this content focused environment. The effect LLS may have on language proficiency in CLIL have so far been neglected in research. The main aims of the study are to investigate (1) which factors predict LLS use; (2) whether or not LLS use predicts positive language proficiency; and (3) how self-efficacy to learn English as a foreign language affects foreign language development. Structural equation models were implemented to control for confounding background variables such as age, gender, socioeconomic status, IQ, L1 and enrollment in CLIL or regular EFL classes.