Potential differences in achievement between female and male students have always been an interesting topic in educational research, as well as having policy and economic implications. This study provides an overview of the so-called "gender gap" in mathematics and science knowledge, based on an in-depth analysis of both extremes of student ability distributions. Evidence underpinning debate on gender inequality in education can be explored by analyzing trends in these distributions over the last 20 years. This new approach to gender gap analysis shows that while the gender gaps that existed 20 years ago have persisted, gender equality in education has increased. The persistent trend of an overrepresentation of male students in the group of high-achievers in both mathematics and science is striking, but male and female students are often also unequally represented at the lower end of the ability distributions. Patterns differ between countries and cycles. In many countries, male students constitute the majority of the lower end of the ability distribution, while in others, more female students are failing to achieve, especially at grade eight. Some countries have shown a reversed inequality trend over the last two decades. With the proposed approach in analyzing gender gaps, differences at the tails of the achievement distributions can be investigated even if the gender distribution is skewed. Policymakers could make use of the approach to closely monitor the development of achievement gaps in their countries and initiate measures to tackle potential causes of inequity, leading to gender inequalities regarding educational achievement.