In the present study, primary school students' ideas concerning the apparent movement of the Moon are investigated. The research was carried out in five primary schools of Athens (Greece) with a sample of forty (40), fifth and sixth grade students. Semistructured interviews were used to gather scientific data and students had the opportunity to provide their answers either orally and/or as a drawing. The results revealed that students think that (a) apparent movement of the Moon takes place always at night and (b) simultaneous presence of the Sun and the Moon occurs mainly around sunset and sunrise. These ideas were found to be barriers for the construction of the scientific view concerning the relative positions of the Sun-Earth-Moon system, since three-quarters of the students have the idea that the Sun and the Moon are at opposite sides of the Earth. (Contains 8 tables and 1 figure.)