This briefing provides a brief background to the International Day of Women and Girls in Science and why it was established by the UN, and examines women and girls in science in the UK. In the UK, women and girls remain underrepresented in many fields related to science, technology, engineering and maths (STEM) throughout the various stages of development. Analysis from WISE, a UK campaign to promote women in STEM, found that women made up 23% of core STEM occupations in 2017. The under-representation of women in STEM is often described as the ‘leaky pipeline’ problem: more women leave scientific fields at each stage of education and career progression than men. Barriers to women pursuing STEM are complex and multi-faceted but studies have found evidence that factors such as gender stereotypes and working cultures play a part. In answers to recent parliamentary questions, the Government identified a number of initiatives which it has created or supported that aim to increase women and girl’s participation in STEM, including the ‘year of engineering’ and the Stimulating Physics Network.