When comparing data derived from tests or questionnaires in cross-national studies, researchers commonly assume measurement invariance in their underlying scaling models. However, different cultural contexts, languages, and curricula can have powerful effects on how students respond in different countries. This article illustrates how the application of the Rasch item response theory (IRT) model (Rasch, 1960) can be used for assessing differences in measurement properties of tests and questionnaires with reference to examples from the field trial analyses for the International Association for the Evaluation of Educational Achievement (IEA) International Civic and Citizenship Education Study (ICCS). It also discusses the general scope and limitations of the analyses undertaken in the context of this study. (Contains 8 notes, 3 tables, and 6 figures.)