"I believe that he/she is telling the truth", "I know about the solar system": what epistemic criteria do students use to distinguish between knowledge and beliefs? If knowing and believing are conceptually distinguishable, do students of different grade levels use the same criteria to differentiate the two constructs? How do students understand the relationship between the two constructs? This study involved 219 students (116 girls and 103 boys); 114 were in 8th grade and 105 in 13th grade. Students had to (a) choose which of 5 graphic representations outlined better the relationship between the two constructs and to justify their choice; (b) rate a list of factual/validated, non-factual/non-validated and ambiguous statements as either knowledge or belief, and indicate for each statement their degree of truthfulness, acceptance and on which sources their views were based. Qualitative and quantitative analysis were performed. The data showed how students distinguish knowledge from belief conceptually and justify their understanding of the relationship between the two constructs. Although most students assigned a higher epistemic status to knowledge, school grade significantly differentiated the epistemic criteria used to distinguish the two constructs. The study indicates the educational importance of considering the notions of knowledge and belief that students bring into the learning situation.