This study presents evidence on the factors that enable witnesses to support victims of intimate partner violence (including to report to the relevant authorities). The aim of the study is to contribute to knowledge among relevant decision makers on viable and effective approaches in European Union (EU) Member States aimed at increasing the role of witnesses, which can be used to improve the level of responses to intimate partner violence. Drawing on desk research in all Member States and in-depth qualitative research in Denmark, Germany, France and Portugal, this study makes a novel contribution in collating existing evidence and presenting new evidence on witnesses' support for victims of intimate partner violence in Member States.