Entrenched socioeconomic inequalities in achievement have been observed in international assessments for decades, with previous research suggesting that these inequalities may be exacerbated by differentiation in the opportunities provided to various social groups. Much previous research in this field has been US-based or subject to measurement issues. This study investigated socioeconomic inequalities in outcomes and opportunities using four cycles of international assessment data across 78 countries. Further, the paper questions whether inequalities in opportunities are related to inequalities in outcomes. The findings indicate global social inequalities in mathematics and science outcomes, while inequality of opportunity was particularly concentrated in economically advanced countries. Little evidence of the perpetuation of socioeconomic inequality in achievement through differentiated educational provision was found.