This article reports on a study that investigated metacognitive knowledge or beliefs about language learning of 470 Turkish EFL learners in secondary education. The primary aims of the study were to explore what beliefs Turkish students in secondary education held about learning English as a foreign language (EFL), how their belief systems were organized and whether there were significant differences in belief systems among learner groups according to variables such as social and school contexts, gender, age and grade level. Popular conceptions of language learning were collected using a structured questionnaire based on Horwitz's Beliefs About Language Learning Inventory (BALLI) (1987). An exploratory factor analysis was performed to identify Turkish EFL learners' patterns of beliefs about language learning. Subsequently, various statistical tests were carried out to find out intra- and inter-group variability in belief categories. The results of this study demonstrated that Turkish EFL learners have a broad range of conceptions both similar to and different from those reported in the current literature. It is further evident that learners' metacognitive knowledge or beliefs about language learning have variability in terms of social and educational contexts, age, gender, and stages of language learning. The study also suggested that curriculum designers and decision-makers as well as language instructors attend to the accumulation of metacognitive knowledge or learner beliefs. (Contains 13 tables.)