This book addresses co-occurring substance use disorders and mental disorders through a comprehensive, practice-oriented, and integrated perspective that reflects the real-life complexity of these conditions. It approaches the subject matter by emphasizing simultaneous treatment of mental health and substance use challenges, recognizing that effective support must involve coordinated efforts across health, social, and welfare services rather than fragmented or sequential care. What stands out with this approach is its strong focus on implementation and collaboration. Instead of presenting isolated theoretical models, the book demonstrates how integrated and holistic treatment can be achieved in everyday practice. Drawing on the combined clinical and academic expertise of its contributors, it highlights concrete service models, interdisciplinary cooperation, and shared competencies that are relevant regardless of professional background or service level. The scope of the book is wide-ranging, covering both individual and system-level perspectives. It addresses assessment and treatment, while also exploring key themes such as housing, employment, somatic health, coercion, user involvement, and recovery-oriented services. This breadth ensures a nuanced understanding of coordinated and multidisciplinary care. The book is aimed at students and professionals within health and social care education and practice. Its content level is accessible yet thorough, making it suitable both as an introductory text and a practical reference.