The chapters in this issue focus on equivalence and comparability issues in international assessments. The authors examine the impact of these issues on the validity of international assessments and the interpretation of results. The chapters were written by individuals with extensive experience in the general methodological area they discuss and who dealt first hand with the specific issues in the Third International Mathematics and Science Study (TIMSS) or in previous studies conducted under the auspices of the International Association for the Evaluation of Educational Achievement (IEA). Each author makes useful recommendations regarding the design, development, and reporting of international assessments to improve comparability of results across countries.