In the International Computer and Information Literacy Study (ICILS) 2018, computational thinking (CT) assessment was an option for participating countries participating. Eight countries and one benchmarking participant participated in the CT assessment. In this chapter, the CT assessment instrument and the proficiency scale derived from the ICILS 2018 test instrument and data are described and the international student results relating to CT are discussed. CT achievement is described across three regions of increasing sophistication from a functional working knowledge of computation as input and output (lower region) through to an understanding of computation as a generalizable problem-solving framework (upper region). Students’ CT achievement varied more within countries than across countries. CT tended to be higher among male students, although statistically significant gender differences were found in only two countries. In one of those countries the difference was in favor of female students and in the other it was in favor of male students. Socioeconomic status was significantly positively associated with student CT achievement. Immigrant background, language background, access to computers at home, and experience with computers were also associated with student CT achievement. In all countries, student CT achievement was strongly associated with student computer and information literacy (CIL) achievement.