This exploratory study examines the beliefs of secondary school students at different academic levels regarding the construction and nature of scientific knowledge and scientific models. To this end, a questionnaire was administered to 151 secondary school students in grades 8-12 (ages 12-18). Items included in the questionnaire relate both to nature, elaboration and validation of scientific knowledge and to nature, role, formulation and validation of scientific models. Based on the scores obtained by students and the analyses of variance undertaken, it can be concluded that: a) students' knowledge about the nature of science and scientific models is not epistemologically appropriate and does not improve the higher academic level is; b) students' ideas about scientific models are significantly better than ideas about the construction of scientific knowledge, regardless of academic level; and c) students tend to overvalue the role of observation and experimentation in the processes of scientific knowledge construction.