Katzmarzyk, along with their research collaborators, summarize the initial findings from the REACT Project ("Return-to-Action after the COVID-19 Pandemic"). The REACT Project is a longitudinal study of growth, motor development, and health behaviors carried out among school children ages 6-10 years from Matosinhos, northern Portugal. The primary objectives of this research were to: (1) evaluate children's growth and motor development following the COVID-19 pandemic, and (2) track the developmental trajectories of fundamental movement skills (FMS) using a novel technology, the Meu Educativo ® app, in their physical education classes. This is the first special issue of the AJHB to provide findings from a single, pandemic-initiated research project. The eight papers in this issue offer an example of the impressive work being done by human biologists to explore the short and longer-term influences of the pandemic on human health and well-being. The REACT Project's focus on evaluating and promoting healthy patterns of physical growth, activity, and motor development in the wake of the pandemic is particularly salient given the importance of these early-life factors for shaping adult health outcomes and disease risks.Sara Pereira et al. (2024b) open this special issue by providing an overview of the objectives, research design, and methodologies used in the REACT study. The thoughtful and rigorous protocol for the study is allowing the researchers to explore the influence of diverse family, school, and neighborhood factors in shaping children's physical growth, movement skills, fitness, and health behaviors. Such rich insights offer to provide valuable information to both parents and educators on how to effectively promote physical activity and healthy growth and development among school-aged children.Papers by Donald Hedeker et al. (2024) and Fernando Garbeloto et al. (2024b) highlight the novel analytic and statistical approaches employed in the REACT research project. Hedeker and colleagues offer a detailed and accessible discussion of the utility of multilevel ordinal logistic models for evaluating longitudinal changes in motor development. Garbeloto and colleagues document the