Among the many troubling aspects of the rising prevalence of obesity in the United States and elsewhere in recent years, the growth of early childhood overweight and obesity stands out. To explore what is known about effective and innovative interventions to counter obesity in young children, the National Academies of Sciences, Engineering, and Medicine's Roundtable on Obesity Solutions held a workshop in October 2015. The workshop brought together many of the leading researchers on obesity in young children to describe the state of the science and potential solutions based on that research. Participants explored sustainable collaborations and new insights into the implementation of interventions and policies, particularly those related to nutrition and physical activity, for the treatment and prevention of obesity in young children. Obesity in the Early Childhood Years summarizes the presentations and discussions from the workshop.