The aim of this review is to examine two factors that may be associated with development of childhood overweight: early feeding, namely exclusive breastfeeding practices; family structure. Findings from the Identification and prevention of Dietary- and lifestyle-induced health EFfects In Children and infantS (IDEFICS) study are presented in the context of the literature. IDEFICS is a multi-centre European study exploring the risks for overweight and obesity in children, which recruited 16,224 children aged 2-9 years from September 2007 to June 2008 at survey centres in Italy, Estonia, Cyprus, Belgium, Sweden, Hungary, Germany and Spain. Among the IDEFICS sample, after controlling for confounders, exclusive breastfeeding for 4-6 months was protective of overweight (including obesity) when compared with children never exclusively breastfed (OR 0·73, 95% CI 0·63, 0·85). Family structure and number of siblings may also be associated with overweight. IDEFICS children without siblings were more likely (OR 1·52, 95% CI 1·34, 1·72) to be overweight than their peers with siblings when controlling for factors related to childhood overweight such as country, parental education, parental weight, maternal age, child's age, birth weight and gender. Both early feeding practices and family structure play a role in the future development of obesity. The impact of breastfeeding on future development of overweight is dependent upon the dose. Exclusive breastfeeding for the recommended 6 months appears to be protective of overweight. Family structure is also an important component and emerging research suggests only children are at increased risk for overweight in comparison with those with siblings. In European countries, approximately 22 million children are overweight. Early dietary exposures, genetic, environmental and social factors have all been proposed as potential causal factors. Two such factors include exclusive breastfeeding and the impact of being an only child. We have investigated these two factors for associations with overweight; our studies, in the context of previous findings, are the focus of this review.