The PISA 2006 assessment more clearly separates knowledge about science from knowledge of science. Knowledge of science refers to knowledge of the natural world across the major fields of physics, chemistry, biological science, Earth and space science, and science-based technology. Knowledge about science refers to knowledge of the means (scientific enquiry) and the goals (scientific explanations) of science. The PISA framework further elaborates on, and gives greater emphasis to, knowledge about science as an aspect of science performance, through the addition of elements that underscore students´ knowledge about the characteristic features of science. The PISA scientific competencies can be thought of as a sequence of strategies students use when solving a problem. First they identify the problem, then apply their knowledge of science to find a solution, and finally interpret and use the results. The three competencies defined in PISA 2006 for science are identifying scientific issues, explaining phenomena scientifically and using scientific evidence. The term `scientific literacy´ used in this report refers collectively to both knowledge about science and knowledge of science.