The present study examines the integration level of the Nature of Scientific Knowledge in secondary school science classes in Greece. The research was designed and organized on the basis of the Cultural-Historical Activity Theory. The activity system of the researchers, who use the proper tools each time, are in network relations with the activity system of Science Education. Major components of the educational system are studied (the curriculum, textbooks, teachers' know-how and teaching methods, school inspectors' viewpoints, students' knowledge) in relation to the nature of scientific knowledge, to ensure valid results. The curriculum and textbook content is decoded, the knowledge of teachers and students is assessed with the use of an internationally validated questionnaire, and interview protocols are analyzed. Research results reveal that the nature of scientific knowledge is included in a small degree in the curriculum and textbooks, teachers refer intuitively to some of these aspects, without assessing the knowledge of students, and, finally, the majority of high-school graduate students have naïve views regarding the nature of scientific knowledge.