The literature on achievement inequality has recently started to focus on the dynamics of the socio-economic achievement gap in cognitive abilities. The main findings come from research in the U.S. revealing that the 90th/10th income achievement gap has widened by about 50% in the last 30 years. This chapter aims to investigate whether there are discernible patterns in the evolution of the achievement gap from a comparative perspective. Using over 15 years of data and 32 countries from the Program for International Student Assessment (PISA), I find that there is considerable variation in the way in which the gap is evolving, with the U.S. and Germany closing at about 50% and 30% in the last 15 years while France is widening at a similar rate. I find that curricular tracking and vocational enrollment explain 40% of the variance in the achievement gap between countries and show that the relationship is conditioned by a strong interaction. Low curricular tracking is associated with a small achievement gap, whereas high levels of curricular tracking is associated with wide achievement gaps. However, once tracking is coupled with high vocational enrollment this can remedy the potential adverse effects and reduce the gap by over 1 standard deviation. I use simulations to show that switching to less curricular tracking can help decrease a country’s SES gap by about 11% while switching to more tracking would increase the achievement gap by about 51% percent.