Although the participation rates of females in science, technology, engineering, and mathematics (or STEM) education is poor in most Western countries, considerable differences across countries exist as well. This may be due to differences in the so-called gender achievement gaps, that is, delays of one sex with respect to the other. The variation in gender gaps in mathematics, science, and reading literacy, both across countries and across schools within countries, is explored in the present study using the PISA data. The results of multilevel analyses show the participation of women in tertiary STEM education to increase as the relative achievements of girls with respect to boys in secondary education improve. When the characteristics of schools and countries are examined in relation to the size of the gender achievement gaps, integrated educational systems are found to be more favourable to the achievement of girls than differentiated educational systems. (Contains 7 tables, 2 figures, and 9 endnotes.)