Gemini is a large population-based study of twins designed to investigate the genetic and environmental influences on obesity risk, weight gain, and early growth. Gemini have followed twins and their families for over a decade, collecting detailed information about the twins’ growth, eating behaviours, appetite, and the home environment. A key aim of the Gemini study is to create a rich resource of data on early childhood exposures that can be used to assess the determinants of long-term health.