The research for teachers' conceptions of the outside educational materials, that they discover and use, is very limited. In addition, there is also particularly limited research comparing teachers' conceptions of the use of educational materials on the basis of the subject they teach. The present paper aims at investigating and comparing primary school teachers' conceptions of the use of outside educational materials in mathematics and science teaching. For the purposes of this research, an electronic questionnaire was developed and completed by 212 primary school teachers in Greece. Data analysis traced primary school teachers' conceptions of whether they use outside educational materials and, if they do, how frequently and in what way they use them, what motivates the teachers to discover outside educational materials and where they discover them, as well as the reasons why they might not use outside educational materials. Furthermore, the differences among teachers' conceptions of the above issues were detected on the basis of the subject they teach (mathematics, science).