More than 1 billion children - half of all children in the world - are exposed to violence every year, in many forms and places, online and off. Whether a target or a witness, a child's exposure to violence has a severe impact on mental health. Such experiences with violence are often traumatic, evoking toxic responses to stress that cause both immediate and long-term physiological and psychological damage, including depression, post-traumatic stress disorder, borderline personality disorder, anxiety, substance use disorders, sleep and eating disorders, and suicide. As the international community begins a Decade of Action to deliver the Sustainable Development Goals (SDGs) by 2030, mental health must be imperative to measures employed to prevent and respond to violence against children and fulfill the promises of the Convention on the Rights of the Child. In support of that process, this publication provides an overview of international evidence on the ways in which violence harms children's mental health, considering both diverse settings and forms of violence, developmental differences in the aftermath, and the most significant risk and protective factors. Highlighting the urgent need for action alongside continued gaps in knowledge of worthwhile solutions, the report also offers existing, effective approaches that should be pursued