In this paper, the focus is upon the rationale for and the nature of the framework for assessing scientific literacy in the OECD/PISA project. Although pilot trials of test materials were conducted in May 1999, at this point it is only possible to report the procedures for analysis and selection of items and not their outcome. The paper begins with a brief overview of the intentions of the programme as a whole, since these set the parameters for the assessment in each of the domains selected for the surveys: reading literacy, mathematical literacy and scientific literacy. This first section attempts to answer questions such as: why another international survey? how does PISA differ from TIMSS? which countries are participating? The second section discusses the interpretation of scientific literacy put forward by the Science Functional Expert Group (SFEG) which was set up to advise on what to assess and how it might be assessed. This interpretation has been agreed by the committee, comprising representatives of the participating countries, which steers the project on matters of policy and ensures adherence to policy decisions as the programme proceeds. The third section describes the different aspects of scientific literacy that are being used to develop assessment units. Finally, the nature of the units is mentioned briefly and examples are given.