The initiative for this review of appropriate timing for complementary feeding of the breastfed infant came from the Consultative Group on Maternal and Young Child Nutrition, an expert supportive group to the UN/ACC Subcommittee on Nutrition. Evidence assessing the duration of breastfeeding adequacy and the advantages and disadvantages of early versus later introduction of complementary foods is reviewed. Several years ago the Protein Advisory Group (PAG) recommended that complementary foods be provided between 4-6 months of age. In later studies, the variability of circumstances in different ecologies were reported to have a significant impact on the ability and desire of a woman to breastfeed and for how long. Hence the appropriate time to provide a breast milk complement is again under examination. Nutritionists, health professionals, and the baby food industry (their pro and antagonists) have renewed their interest in this issue, but differences in opinion and distorted claims among the interested groups have left many people confused. The appropriate time for the introduction of complementary feeding varies considerably in different sociocultural settings and among different socioeconomic groups. In developed countries very early complementation is usual not because of nutritional need, but because of social pressures, availability of convenient alternatives, and other considerations. In developing countries, early complementation may be needed in circumstances with a high prevalence of maternal malnutrition that may contribute to low milk output. Hazards accompany both too early and too late complementation. If a complement is introduced too early, there is the danger of diarrheal disease and food allergies. There is also an economic burden on poor families to purchase complementary food. In some underprivileged areas, cultural patterns in which the introduction of complementary food is too much delayed is the primary cause of faltering growth and malnutrition. Most reviewed data indicate that by 6 months nearly all infants are in need of complementary foods in order to maintain normal growth. There are individual exceptions under ideal conditions and with very high maternal milk yields. With a few individual exceptions, there are no short or longterm advantages in introducing complementary foods before 4 months. Normal growth of entirely breastfed infants, as opposed to that of bottlefed infants on which many of the present growth standards are based, should be studied further.