This book is for youth workers, social workers, mental health staff, specialist teachers, family support workers, and so on, whose clients present with comorbidity, risk, and difficulty accessing mainstream services. It describes inevitably stressful, unsettling work, providing effective help in complex helping systems. An innovative response emerges, building on adaptive (evidence-based) mentalization-based theory and practice. Uniquely, AMBIT applies mentalizing not only directly, in work with clients, but also in work: (a) with the team, (b) with wider (often “dis-integrated”) networks, and (c) creating cultures of learning and radical transparency. AMBIT is as much an improvement system for teams as a “therapy”—strengthening team identity and coherence, and supporting a wider community of practice. Linking evidence-based practice to practice-based evidence, the book concludes with impact descriptions from some of the nearly 200 AMBIT-trained teams, a client’s perspective, and a challenging analysis of systems of care pointing toward the need to create more mentalizing systems.