The American Medical Informatics Association (AMIA) defines the term biomedical informatics (BMI) as: The interdisciplinary field that studies and pursues the effective uses of biomedical data, information, and knowledge for scientific inquiry, problem solving and decision making, motivated by efforts to improve human health. This book: Applied Interdisciplinary Theory in Health Informatics: A Knowledge Base for Practitioners, explores the theories that have been applied in health informatics and the differences they have made. The editors, all proponents of evidence-based health informatics, came together within the European Federation of Medical Informatics (EFMI) Working Group on Health IT Evaluation and the International Medical Informatics Association (IMIA) Working Group on Technology Assessment and Quality Development. The purpose of the book, which has a foreword by Charles Friedman, is to move forward the agenda of evidence-based health informatics by emphasizing theory-informed work aimed at enriching the understanding of this uniquely complex field. The book takes the AMIA definition as particularly helpful in its articulation of the three foundational domains of health informatics: health science, information science, and social science and their various overlaps, and this model has been used to structure the content of the book around the major subject areas. The book discusses some of the most important and commonly used theories relevant to health informatics, and constitutes a first iteration of a consolidated knowledge base that will advance the science of the field.