Three studies are reported that investigated the relationship between secondary school students' physics-related epistemological beliefs and physics conceptual understanding. Study 1 involved the development of a Greek Epistemological Beliefs Evaluation Instrument for Physics (GEBEP) which was administered to 394 students (10th graders). Study 2 investigated the hypothesis that physics epistemological sophistication as measured by the GEBEP is a good predictor of physics understanding. The participants were selected from the 394 students who participated in Study 1. More specifically we selected the 10% (38) students with the highest scores in the GEBEP (high epistemological sophistication group, HES) and the 10% (38) students with the lowest GEBEP scores (low epistemological sophistication group, LES) and measured their understanding of Newton's three laws using the Force and Motion Conceptual Evaluation instrument (FMCE) developed by Thornton and Sokoloff (1998). The results showed that the HES group had significantly higher scores in the FMCE than the LES group. Regression analysis showed that beliefs regarding the "Construction and Stability" of physics knowledge and the "Structure" of physics knowledge were good predictors of physics understanding. Study 3 re-examined the same hypothesis on a new independent sample of students. The results based on the entire sample, showed again that beliefs regarding the "Construction and Stability" of physics knowledge predicted physics understanding. Overall, the results suggest that sophisticated physics-related epistemological beliefs are necessary but not sufficient for physics understanding and point to the importance of taking them into consideration in physics education.