Connections and consequences in complex systems: Insights from a case study of the emergence and local impact of crisis resolution and home treatment services

► Complexity ideas are used to frame an examination of the connections between levels of organisation in health and social care. ► The wider, system, consequences of service development in the specific context of mental health are explored. ► Data were generated in a case study of the emergence and impact of a crisis resolution and home treatment team in Wales, UK. ► Whilst the treatment team provided a high-quality service it also triggered significant effects across the local system. ► Lessons are drawn on evidence and policy, local contexts and system interfaces, and on anticipating the unexpected.