In close collaboration with the participating countries, the international study center (ISC) developed an international English version of the ICCS assessment and questionnaires. These materials were subsequently translated and adapted by countries to their languages of instruction. Throughout this translation and adaptation process, the overarching aim was to create high-quality instruments that were internationally comparable yet also appropriate to each country’s national context and education system. The ICCS instruments were administered in 31 languages, the most common being English and Spanish. Even when the language of testing was English, adaptations were still required to suit the cultural setting and the version of English being used. Because all countries participating in the Latin American module required Spanish as the assessment language, the instrument was developed in this language for use and adaptation by countries participating in this module. Given that high-quality translations were crucial to the quality of the ICCS data, all national instruments were subject to a stringent international verification process that included a thorough review of the adaptations, translation, and layout. This chapter provides an overview of this process.