Achievement in mathematics of eighth-grade students is modeled as a function of within-school, between-school and cross-country differences. The data were obtained from 217,728 students, within 7,216 secondary schools, in 48 countries, who participated in the 2007 Trends in International Mathematics and Science Study. Multilevel analysis showed that out of the total variance in mathematics achievement, 40.39%, 20.61%, and 38.99% were accounted for within-school, between-school-within-country, and cross-country differences, respectively. Mathematics self-concept followed by socioeconomic status was the strongest predictor of achievement at the student level. At the school level, school location yielded the strongest link to achievement, while at the country level socioeconomic status was the main predictor of national mathematics average.