The paper will deal with the language aspects of scientific literacy in the compulsory school. Scientific literacy is a main goal for most compulsory science education. It is a very broad concept with many diverse definitions. The paper will analyse some widely used definitions and curriculum projects and point out the increased focus these have put on the linguistic aspects of the science practice in the classroom in the form of argumentation, discussion, making inferences, drawing conclusions etc. These general considerations will be illustrated with data from a Danish research project about the PISA science test. The science part of PISA aims at testing the scientific literacy of 15 year old children. The research project has compared the performance of 130 students participating in the PISA science test with their scientific literacy measured in a more socio-cultural setting than PISA does. As a special aspect of this research, the students’ ability to explain and talk using the science vocabulary was investigated. Based on these empirical data of Danish secondary students’ discourse competence within science, the paper will provide some guidelines for how science literacy can be supported and improved through enhanced emphasis on the scientific discourse within the classroom.