This article introduces the essential features of the science component of 2006 Program for International Student Assessment (PISA). Administered every 3 years, PISA alternates emphasis on Reading, Mathematics, and Science Literacy. In 2006, PISA emphasized science. This article discusses PISA's definition of scientific literacy, the three competencies that constitute scientific literacy, the contexts used for assessment units and items, the role of scientific knowledge, and the importance placed on attitude toward science. PISA 2006 included a student test, a student questionnaire, and a questionnaire for school administrators. The student test employed a balanced incomplete block design involving thirteen 30-minute clusters of items, including nine science clusters. The 13 clusters were arranged into thirteen 2-hour booklets and each sampled student was assigned one booklet at random. Mean literacy scores are presented for all participating countries, and the percentages of OECD students at the six levels of proficiency are given for the combined scale and for the competency scales. (Contains 2 figures, 12 tables, and 1 note.)