This book discusses and analyses how ecological models and approaches could contribute to improve human health. In the first section of the book, the evolution of human thought will be redrawn, from ancient conceptions of the world up to the most recent ecological models. The second section of the book presents a review of ecological theories, models, and approaches relevant to the field of health. The text proceeds in an historical perspective and from general to specific: generic theoretical referents followed by referents specific to a field of practice, a discipline, a population, or a situation. Numerous examples are provided. In the third section, common characteristics of ecological theories, models and approaches are described: 1) the consideration of personal, environmental, and occupational components, and the consideration of interactions between them; 2) the positive vision of health; 3) the focus on empowerment, self-determination, and self-regulation of individuals, and finally, 4) the fact that ecological interventions occur mostly in natural environments, where the person interacts with his or her environment and performs his or her occupations, rather than in institutions or hospitals. The fourth section deepens the concepts of environment and interactions, since these concepts were to date less defined in previous publications. New theoretical referents are proposed for these two elements. The fifth section presents the results of numerous studies with the aim of providing scientific evidence in support of ecological models and approaches. These studies are grouped according to the targeted components, which are the person, environment or occupation, as well as the interactions between these components. Finally, the sixth section brings a different perspective. Some fundamental ecological mechanisms of interaction are analysed according to human health and to the current context: demographic changes, climate change, depletion of resources, new diseases, pandemic risks and others. These events will, without a doubt, provoke a budgetary crisis which governments and health care systems will have to face; thus supporting the urgency to adopt ecological approaches. Many research questions are proposed to improve knowledge in the field of human ecology in this section. Concrete suggestions are also identified in order to improve population health. The conclusion presents a lucid and global synthesis of the situation. Humanity is at a stage never reached before, notably because of the presence of so many individuals on earth. Furthermore, human beings possess powerful abilities and technological possibilities but also the awareness and the responsibility which these elements confer to us. All health professionals are called to contribute to necessary efforts in favour of population health, now and for future generations, in a sustainable development perspective.