In health research, approaches based on biomedical and epidemiological studies fail to capture the complexity of population health interventions. Yet it is essential to study them using approaches adapted to their complexity, the plurality of their mechanisms, and the diversity of local actors and contexts. This is the purpose of population health intervention research (PHIR), as summarized in this book. Through conceptual and methodological explanations, and illustrations of numerous examples from around the world, the authors aim to address a need for training in this scientific approach among researchers and, more broadly, healthcare professionals, by examining essential issues of effectiveness, equity, relevance, implementation, and scaling-up.