International large-scale assessment studies comparing the output and outcomes of education in different countries have become more and more prominent over the past 20 years. Ever since, these studies have provided a meaningful and influential empirical reference for educational policymaking. This chapter elaborates the history and characteristics of, as well as some theoretical frameworks used in, international large-scale assessments. We distinguish and discuss different types of large-scale assessments in the current educational landscape. Large-scale assessments are especially valuable in their contribution to monitoring and analysing educational systems. However, there are various challenges involved in combining test data and social background variables as well as the characteristics of the educational environments. This chapter discusses how best to deal with these problems in order to allow statements to be made on the quality of educational systems.