The aim of this study was to investigate how the best interest of the child was considered and reasoned with in cases where one parent has been using violence against the other parent and child has been exposed to this. The focus of our study was to see how the different units in social services cooperate for the best interest of the child and also how they reason their own assessment in these cases. The empirical data of this study consists of 8 qualitative interviews with social workers in family law or child protection services. We have chosen to base our study with three scientific theories, which are: collaboration theory, interorganizational organization theory and the theory of professions. The outcome of this study has been coded and themed in diverse steps to pinpoint the fundamental categories. These categories are: factors of importance in collaboration and factors that affect what is considered to be the best interest of the child. Based on our results and the empirical data studied, we have drawn a conclusion that there are many different and complex factors that can influence the decision making of a child's contact with an abusive parent. The interpretation and collaboration can be influenced by different factors such as the goals and purpose of the unit. It can be stated through our study that there is a positive view of collaboration between family law and child protection services. However, it can also be stated that the collaboration does not occur to the same extent as one would have liked within the units and that this could be due to various factors. We have also noticed that there has not been a lot of research on our topic regarding the interaction between family law and child protection services, especially regarding cases of violence. There is a need for more research regarding an internal collaboration within the social services that can be implemented as there are shortcomings in practice that have emerged in our study. We believe that the social workers in the social services and the families in question would have benefited from a more comprehensive research on how to interact with each other for the best interests of the child.