The first 1,000 days – conception through age 2 – represents an important period for the development and prevention of childhood obesity. This study reviews existing evidence from interventions occurring in the first 1,000 days that included prevention of childhood overweight or obesity as an outcome, identifies gaps in current research, and discusses conceptual frameworks and opportunities for future interventions. The results of this study are based on 34 articles representing 26 unique completed interventions, published between January 1, 1980 and December 12, 2014, as well as 46 identified ongoing trials. Nine of the completed interventions were found to be effective. Across both completed and ongoing interventions, the majority targeted individual-level behaviors and many were confined to clinical settings; few targeted early-life systems and policies that may impact childhood obesity. Obesity interventions may have the greatest preventive effect if begun early in life, yet few effective interventions in the first 1,000 days exist, and many target individual-level behaviors of parents and infants.