This brief considers how women are essential to the successful establishment and reconstruction of state security systems during periods of post-conflict reconstruction. The brief uses examples from South Africa, Colombia, Nepal, Guatemala, Sudan, and Iraq to help define what security sector reform (SSR) is, who designs it, who implements it, and why gender mainstreaming in SSR processes is so essential. Furthermore, the brief provides a summary of current international action on security security reform and provides a basic checklist for women peacebuilders looking to make an impact on security systems.