The purpose of my study is to look into how teachers work with children and aggression, and how they work with Aggression Replacement Training, ART in Swedish schools. I also want to get the backgrounds of why a child’s behaviour can become aggressive. To reach my purpose I’ve chosen to use a qualitative method. I have chosen four teachers who all have great experience of working with aggressive children, and the ART pedagogic. I have chosen thirteen questions to ask the teachers who I interviewed two at a time and I used a tape recorder during the interviews as a research. The literature study contains research about theories of aggression, children and ART- pedagogic. My results show that it can bee two causes behind the aggressive behaviour of children. One is trough heredity and the other has environmental causes. Through my own research I have found that there is a lack of research in this area. I believe that if we are going to help children with aggressive behaviour we need to put effect into the small children and the causes behind this behaviour. I have also found that the original version of ART is difficult to use in Swedish schools. However, the edit Swedish version of ART fits better in the Swedish school.