Parents, students, teachers, governments and the general public – all stakeholders - need to know how well their education systems prepare students for real-life situations. Many countries monitor students’ learning to evaluate this. Comparative international assessments can extend and enrich the national picture by providing a larger context within which to interpret national performance. They can show what is possible in education, in terms of the quality of educational outcomes as well as in terms of equity in the distribution of learning opportunities. They can support setting policy targets by establishing measurable goals achieved by other systems and help to build trajectories for reform. They can also help countries work out their relative strengths and weaknesses and monitor progress.