The beliefs people may have about the nature of knowledge and the procedure of knowing have a significant impact on conceptual development. We claim that personal epistemology is a system of interrelated dimensions that are developed in their own pace and they are domain specific. In the current study we were keen at investigating children's and adolescents personal epistemology on Physics. Using the Conley's et al. (2004) questionnaire for the measurement of the personal epistemology, 280 children and adolescents 8.5, 11.5 and 14.5-years-old participated in the study. The results confirmed the 4 supposed subscales of the questionnaire. Greek students hold sophisticated epistemological beliefs about the development of knowledge and the justification of knowing. Their epistemological beliefs about the source of knowing and the certainty of knowledge are found to be less sophisticated. Educational implications for teaching Physics are discussed.