Although homework is generally believed to be an important supplement to in-school learning, research has not yet fully clarified the relationship between homework and achievement. This cross-cultural study analyzed the relationship between homework time and mathematics achievement drawing on data from 231,759 students in 9,791 schools and 40 countries who participated in the Programme for International Student Assessment (PISA) 2003. Multilevel analyses found a positive association between school-average homework time and mathematics achievement in almost all countries, but the size of the association decreased considerably once socioeconomic background and school track were controlled. At the student level, no clear-cut relationship was established between homework time and achievement across the 40 countries. The results highlight the need to use multilevel analyses and to control for confounding variables in homework research. (Contains 7 tables, 2 figures, and 4 notes.)