Collaboration between normally separate agencies involved in cases of child mental health, and those supporting their participation and inclusion in school settings, is being increasingly promoted as the answer to intervening in a more ecologically valid and responsive manner. Yet a clear-cut evidence base supporting interagency collaboration in practice is hindered by wide variation in how collaboration is defined, how collaborative initiatives are established and run, and how the success of widely varying programs is interpreted or evaluated. Despite this variation, some strong indicators of success continue to appear. An overview of the literature on interagency collaboration in the context of child mental health, special education, and wellbeing is presented, including risks, benefits, and indicators of success.