Technical Abstract: The International Research on Infant Supplementation (IRIS) studies, performed in 4 developing country settings across the world and presented in this supplement, show that the use of a daily multiple micronutrient supplement during infancy is more efficacious for improving micronutrient status, anemia, and child weight growth than a daily iron supplement or a multiple micronutrient supplement. Neither iron nor multiple micronutrient supplementation prevented growth faltering and, although in the pooled analysis the daily multiple micronutrient supplementation produced a small breaking effect on weight-growth faltering compared with placebo, length-growth faltering was the same in all groups. Infants receiving the daily multiple micronutrient supplement showed improved concentrations of circulating hemoglobin and of plasma ferritin, zinc, retinol, riboflavin, homocysteine, and tocopherol, whereas daily iron supplements and weekly multiple micronutrient supplements only improved hemoglobin and plasma ferritin concentrations. The morbidity data were not collected in comparable ways across countries, but in those country studies that reported morbidity, there was no difference between any supplementation group and placebo.