Hardly a week goes by without deadly acts of violence between partners making headlines. In many cases, these acts have a history of violence - domestic violence. The topic is an integral part of the daily work of the police. It places high demands on police action in terms of danger prevention and criminal prosecution, particularly in connection with the guiding principle "Whoever strikes, must leave." This teaching and study letter provides a concise overview of the essential foundational knowledge required for police action in cases of domestic violence. The first part describes the current state of knowledge on the subject, taking into account European, national, and country-specific studies, statistics, and data. It compactly conveys the important knowledge necessary for assessing the police situation regarding the nature and extent of domestic violence.