Female students are much less likely than their male counterparts to take A levels in certain STEM (Science, Technology, Engineering and Maths) subjects. This is problematic for gender equality but also limits the country's capacity to address the growing national STEM skills shortage. This research was commissioned to design and test the effectiveness of two sets of interventions based on behavioural insights theory, to increase the number of high-achieving female students choosing STEM subjects at A Level. This report also seeks to make recommendations based on the efficacy of the interventions to inform further research and more broadly, to help develop targeted actions to improve diversity in STEM participation. [This report was written by the Department for Education, Behavioural Insights Team.]