In the field of international development, decades of evidence of women’s positive impact on socioeconomic outcomes has changed the way governments, donors, and aid organisations do their work. This shorting briefing suggests that same cannot be said for the field of peace and security, where women have been thoroughly and consistently excluded. Despite a crescendo of calls for women’s participation in decision making surrounding peace and security over the last two decades, change has been slow to follow. It seems that the full impact of women’s participation on peace and security outcomes remains poorly understood in this field. However a recent increase in quantitative and qualitative research has provided the potential to transform this. In outlining the existing data, this brief shows how women’s inclusion helps prevent conflict, create peace, and sustain security after war ends – drawing on examples from around the world.