Satisfactory collaboration regarding children and youth in need of a comprehensive support is particularly important. Despite extensive research on collaborative work, knowledge of long term development of the collaboration process is lacking. The present thesis concerns inter-organizational collaboration within the framework of a former Swedish policy effort – focusing collaboration between schools, social services, police and the child and youth psychiatry – for the benefit of children and young people in distress or at risk. Applying a critical realist perspective, the overall aim of the thesis is to describe how former collaboration projects develop over time, and to identify significant mechanisms within this development. Through three questionnaire studies, the collaborative process development within the same collaborative settings is described (n=66) over a period of close to seven years. Estimations of 58 collaboration quality indicators within three categories *rules and regulations, *structural aspects and *shared perspectives/ consensus were collected at baseline in 2008, after one year at the final project stage in 2009, as well as five years after the project period (and the policy effort) ended, in 2014 (n=38). Two developmental trends occur: I) an overall positive trend and II) a negative trend on a comprehensive level. I) Collaboration on the target group has increased over time, are mainly incorporated into permanent organizational structures and is judged to have worked well/very well over time. II) Overall deteriorations of high estimates of the 58 quality indicators for collaborations is seen over the five year period, following the project period. However, less dramatic changes is noted on quality indicators concerning shared perspectives/consensus than on matters regarding rules and regulations and structural aspects. Five mechanisms of particular importance for the collaboration development are identified: anchoring, holistic perspectives, engagement, knowledge and clarity.