This chapter provides an introduction to the book National Policies and Local Challenges – Organizing Addiction and Mental Health Services. We are interested in how and the extent to which national policy regarding services for people with complex needs, such as people with addiction and mental health problems, is implemented and how these services are organized at the local level. We study addiction and mental health in terms of policy, services and work based on local development processes and national and regional frameworks. The processes observed in our studies can in many ways be said to “mirror” how the modern welfare state addresses other complex problems. Decentralization, user participation and coordination are three key characteristics of development that have significant implications for the design of the service field and set the framework for daily work in local services. The main question underlying our research is: How are governmental policies structured and implemented? This chapter also provides an overview of related questions that frame the various book chapters, followed by a discussion of the book’s structure with a summary of each chapter.