Adherence to long-term therapies is perhaps one of the most important issues that deserves the attention of all stakeholders in the health care field, including doctors, nurses, pharmacists, all health care providers, but also the pharmaceutical industry, health authorities and payers. Everyone agrees that patient adherence, especially for those with chronic diseases, is far from optimal, with catastrophic consequences for the effectiveness of care, which has made more progress in the last 50 years than in the 50 centuries before.The originality of this book relies on the fact that it approaches the problem of adherence in a holistic way, going beyond the term adherence, which was itself already an advance over that of compliance. It proposes replacing it with a more general term, that of engagement. This concept reinforces the involvement of the patient, becoming a true agent in the management of his or her disease. In particular, it is necessary to distinguish between the different phases of this engagement, at the time of treatment initiation, implementation and, last but not least, persistence.This book will be of interest to all care providers. Its ambition is to clarify through theoretical models the mechanisms of patient engagement in treatment, so that practical approaches can be identified to improve it, giving both general indications and specific applications to different groups of patients. An entire section of the book is devoted to care in different contexts: adolescence, the elderly, vulnerable people, the effect of social deprivation cognitive deficits and mental illness. At a time when digital technology and artificial intelligence are transforming the practice of medicine, this book shows their promises and limitations with remarkable intelligence (Chapter 30). Inevitably, the new demands of patients to become agents of their treatment require the development of the concept of shared medical decision making and Chapter 28 devoted to it is especially brilliant. It is this sharing which really defines the concept of engagement, which makes it inseparable from patient education, which truly finds its full meaning in engagement. It is through this process that trust is generated, which is the condition for real patient engagement. It requires that care providers become aware of the human nature of patients, combining the greatness of complex thought with the presence of cognitive and emotional bias. This book therefore sets out the conditions for the real implementation of a person-centered medicine.—Professor Gérard Reach, Professor of Edocrinology and Metabolism, Paris 13 University & Head of the Endocrine Department, Avicenne Hospital, Bobigny, France