Girls’ under-representation in science, technology, engineering and mathematics (STEM) education is deep rooted and puts a detrimental brake on progress towards sustainable development. We need to understand the drivers behind this situation in order to reverse these trends. Cracking the code: Girls’ and women’s education in science, technology, engineering and mathematics provides a global snapshot of this under-representation, the factors behind it and examples of how to improve the interest, engagement and achievement of girls in these fields. Both education and gender equality are an integral part of the 2030 Agenda for Sustainable Development, adopted by the United Nations General Assembly in 2015, as distinct Sustainable Development Goals (SDGs) but also as catalysts for the achievement of all other SDGs.