[Complexity: concept and challenges for public health interventions]

Introduction: Since 2000, the notion of “complex interventions” has been emerging in the health research field. “Complex interventions” and “complexity” are commonly used terms, but they are generally not defined. Conceptual ambiguities persist concerning the notion of complexity. The objective of this exploratory review is to characterize the notion of complexity: What is complexity? Where does this notion come from and what does it cover? What are the consequences of complexity in the health field?Methods: To clarify the concept of complexity, a narrative review was conducted in the fields of humanities and social science, managerial economics, psychology and healthcare.Results: The concept of complexity, that can be attributed to Edgar Morin, has been the subject of appropriations, adaptations, and operations in multiple areas. Complexity consists of understanding the factors influencing individual decisions. In the field of healthcare, the concept of complexity is used more pragmatically and is defined by objective characteristics of interventions (defined as complex) or their contexts for the practical purposes of evaluation.Discussion: The notions of complexity and complex interventions have implications for researchers and users of the results of research. In particular, the notion of complexity is designed to provide a better understanding of the mechanism of effectiveness of interventions, support transferability and use by actors and decision-makers.