This paper examines the extent that material, social, and cultural resources and schools account for the relationship between socioeconomic background and student achievement among 15-year-olds in 30 countries. Generally, cultural factors play a more important role in most countries although in a small minority of countries, material resources have a substantial impact. Most often, social resources have little impact. In many countries, educational differentiation--that is, school tracks and school types, and curriculum tracking within schools--mediates the relationship between socioeconomic background and student achievement. Countries with highly tracked systems tend to show stronger relationships. On average, over 60% of the effect of socioeconomic background on achievement is accounted for by these factors. These findings are independent of whether achievement in reading, mathematics, or science is examined. The implications of this study for reducing socioeconomic inequalities in education are discussed.