This publication is a child of love of one rather specialized sector in education - that of acquiring and using empirical information as a basis for monitoring and studying education with the speciai ambition of making such information both meaningful and powerful, and its use dynamic. This pursuit has a distinguished past, an intriguing present, and a challenging future. In particular, the present publication has been inspired by two major approaches to these ideals: The work of the IEA, the International Association for the Evaluation of Educational Achievement on comparative educational research, which started in the 1950s and is going on. The other and more recent one is the OECD/CERI project INES for developing and acquiring Indicators of National Education Systems, especially its strategy Network A, which aims at collecting student learning outcomes data on a regular basis. We start discussing educational system assessment from various perspectives - both national and international, and as regards both educational research and policy. This is no more a completely open field, but one in which certain premises and traditions have already been established. At the same time, several issues, problems and challenges are still waiting for further discussion and solutions.