BACKGROUND: Undernutrition contributes to five million deaths of children under five each year. Furthermore, throughout the life cycle, undernutrition contributes to increased risk of infection, poor cognitive functioning, chronic disease, and mortality. It is thus important for decision‐makers to have evidence about the effectiveness of nutrition interventions for young children. OBJECTIVES: Primary objective 1. To assess the effectiveness of supplementary feeding interventions, alone or with co‐intervention, for improving the physical and psychosocial health of disadvantaged children aged three months to five years. Secondary objectives 1. To assess the potential of such programmes to reduce socio‐economic inequalities in undernutrition. 2. To evaluate implementation and to understand how this may impact on outcomes. 3. To determine whether there are any adverse effects of supplementary feeding. SEARCH METHODS: We searched CENTRAL, Ovid MEDLINE, PsycINFO, and seven other databases for all available years up to January 2014. We also searched ClinicalTrials.gov and several sources of grey literature. In addition, we searched the reference lists of relevant articles and reviews, and asked experts in the area about ongoing and unpublished trials. SELECTION CRITERIA: Randomised controlled trials (RCTs), cluster‐RCTs, controlled clinical trials (CCTs), controlled before‐and‐after studies (CBAs), and interrupted time series (ITS) that provided supplementary food (with or without co‐intervention) to children aged three months to five years, from all countries. Adjunctive treatments, such as nutrition education, were allowed. Controls had to be untreated. DATA COLLECTION AND ANALYSIS: Two or more review authors independently reviewed searches, selected studies for inclusion or exclusion, extracted data, and assessed risk of bias. We conducted meta‐analyses for continuous data using the mean difference (MD) or the standardised mean difference (SMD) with a 95% confidence interval (CI), correcting for clustering if necessary. We analysed studies from low‐ and middle‐income countries and from high‐income countries separately, and RCTs separately from CBAs. We conducted a process evaluation to understand which factors impact on effectiveness. MAIN RESULTS: We included 32 studies (21 RCTs and 11 CBAs); 26 of these (16 RCTs and 10 CBAs) were in meta‐analyses. More than 50% of the RCTs were judged to have low risk of bias for random selection and incomplete outcome assessment. We judged most RCTS to be unclear for allocation concealment, blinding of outcome assessment, and selective outcome reporting. Because children and parents knew that they were given food, we judged blinding of participants and personnel to be at high risk for all studies. Growth. Supplementary feeding had positive effects on growth in low‐ and middle‐income countries. Meta‐analysis of the RCTs showed that supplemented children gained an average of 0.12 kg more than controls over six months (95% confidence interval (CI) 0.05 to 0.18, 9 trials, 1057 participants, moderate quality evidence). In the CBAs, the effect was similar; 0.24 kg over a year (95% CI 0.09 to 0.39, 1784 participants, very low quality evidence). In high‐income countries, one RCT found no difference in weight, but in a CBA with 116 Aboriginal children in Australia, the effect on weight was 0.95 kg (95% CI 0.58 to 1.33). For height, meta‐analysis of nine RCTs revealed that supplemented children grew an average of 0.27 cm more over six months than those who were not supplemented (95% CI 0.07 to 0.48, 1463 participants, moderate quality evidence). Meta‐analysis of seven CBAs showed no evidence of an effect (mean difference (MD) 0.52 cm, 95% CI ‐0.07 to 1.10, 7 trials, 1782 participants, very low quality evidence). Meta‐analyses of the RCTs demonstrated benefits for weight‐for‐age z‐scores (WAZ) (MD 0.15, 95% CI 0.05 to 0.24, 8 trials, 1565 participants, moderate quality evidence), and height‐for‐age z‐scores (HAZ) (MD 0.15, 95% CI 0.06 to 0.24, 9 trials, 4638 participants, moderate quality evidence), but not for weight‐for‐height z‐scores MD 0.10 (95% CI ‐0.02 to 0.22, 7 trials, 4176 participants, moderate quality evidence). Meta‐analyses of the CBAs showed no effects on WAZ, HAZ, or WHZ (very low quality evidence). We found moderate positive effects for haemoglobin (SMD 0.49, 95% CI 0.07 to 0.91, 5 trials, 300 participants) in a meta‐analysis of the RCTs. Psychosocial outcomes. Eight RCTs in low‐ and middle‐income countries assessed psychosocial outcomes. Our meta‐analysis of two studies showed moderate positive effects of feeding on psychomotor development (SMD 0.41, 95% CI 0.10 to 0.72, 178 participants). The evidence of effects on cognitive development was sparse and mixed. We found evidence of substantial leakage. When feeding was given at home, children benefited from only 36% of the energy in the supplement. However, when the supplementary food was given in day cares or feeding centres, there was less leakage; children took in 85% of the energy provided in the supplement. Supplementary food was generally more effective for younger children (less than two years of age) and for those who were poorer/ less well‐nourished. Results for sex were equivocal. Our results also suggested that feeding programmes which were given in day‐care/feeding centres and those which provided a moderate‐to‐high proportion of the recommended daily intake (% RDI) for energy were more effective. AUTHORS' CONCLUSIONS: Feeding programmes for young children in low‐ and middle‐income countries can work, but good implementation is key.