This paper argues that international studies in the area of educational effectiveness are important for different reasons. It will increase the variation with respect to input process and output and it will show contextual differences between countries with respect to educational effectiveness. Furthermore, in international studies knowledge and experience in research with respect to the development of instruments, design, methodology, and statistical techniques can be shared. It is argued that international research especially needs a theoretical framework. This frame can be used not only to design research but also to understand the results and to explain them. In this way, international comparative research is not restricted to interesting pictures of education in several countries but can contribute to further understanding about what works in education and why.