This study examines Turkish pre-service science teachers' understanding of science concepts and their confidence in its teaching. A total of 299 senior science education major students participated in the study. Data collection instruments included the Science Concepts Test, and the Science Teaching Efficacy Belief Instrument. Although findings of the study indicated that majority of the participants held misconceptions concerning fundamental science concepts, they generally felt confident about teaching it. The relationship that might exist among pre-service science teachers' confidence in their effectiveness in teaching and the number of the science courses completed in university is examined.