Gemini is a cohort study of young twins in the United Kingdom designed to assess genetic and environmental influences on early childhood weight trajectories with a focus on infant appetite and the family environment. A total of 2402 families with twins born in England and Wales between March and December 2007 agreed to participate and returned completed baseline questionnaires. The sample includes 1586 same-sex and 816 opposite-sex twins. The study is currently funded for 5 years of follow-up, but is planned to continue into early adolescence and beyond, pending funding. With current funding of the study, families will be followed up when twins are: 8 months old (baseline), and then at 15, 20, 24, 36 and 48 months of age. Gemini is in its early stages, with baseline and first follow-up data collection completed. This is the first twin cohort to focus on childhood weight gain with detailed and repeated measures of children's appetite, food preferences, activity behavior and parental feeding styles, alongside detailed and repeated collection of anthropometrics. This article reviews the rationale for the Gemini study, its representativeness and the main measures.