Obesity in children remains a major public health problem, with the current preva-lence in youth ages 2–19 years estimated to be 19.7%. Despite progress in identify-ing risk factors, current models do not accurately predict development of obesity inearly childhood. There is also substantial individual variability in response to a givenintervention that is not well understood. On April 29–30, 2021, the National Insti-tutes of Health convened a virtual workshop on“Understanding Risk and CausalMechanisms for Developing Obesity in Infants and Young Children.”The workshopbrought together scientists from diverse disciplines to discuss (1) what is knownregarding epidemiology and underlying biological and behavioral mechanisms forrapid weight gain and development of obesity and (2) what new approaches canimprove risk prediction and gain novel insights into causes of obesity in early life.Participants identified gaps and opportunities for future research to advance under-standing of risk and underlying mechanisms for development of obesity in early life.It was emphasized that future studies will require multi-disciplinary efforts acrossbasic, behavioral, and clinical sciences. An exposome framework is needed to eluci-date how behavioral, biological, and environmental risk factors interact. Use of novelstatistical methods may provide greater insights into causal mechanisms.