This paper is based on fieldwork undertaken in the course of an evaluation of the science curriculum of the lower secondary level in Spain. This level of the Spanish school system has experienced an intense process of reform over the last 10 years. The paper analyses the process of change affecting the subject of Natural Sciences in the context of an institutional reform in which the roles of teachers and students are undergoing important modifications. These changes foreshadow three issues related to the epistemology of the Natural Sciences: its organization in terms of students' learning; the propedeutic of the subject; and the understanding of science as related to technology and its societal impact. The traditional academic science of the Spanish school is not the reference point for this 'new' subject, and this has important consequences for teachers in terms of both the curriculum and their relation with the subject in their own background.