Complementary food is needed when human milk (or infant formula) alone is no longer sufficient for nutritional reasons. The timing of introduction needs to be determined on an individual basis although 6 months of exclusive breastfeeding can be recommended for most healthy term infants. Solid foods are intended to 'complement' ongoing breastfeeding with those dietary items whose intake has become marginal or insufficient. Both breastfeeding and complementary feeding can have direct or later consequences on health. Possible short-term health effects concern growth velocity and infections while possible long-term effects may relate to obesity, cardiovascular disease, autoimmunity (celiac disease and type 1 diabetes) and atopic disorders. For most of these it is impossible on the basis of the available evidence to conclude on the age when risks related to the start of complementary feeding are lowest or highest, with the possible exception of infections and early growth velocity. For undesirable health consequences, whilst potential mechanisms are recognized, the evidence from mostly observational studies is insufficient and requires more and prospective research. While the 6-month goal is desirable, introduction of suitable complementary food after 4 completed months with ongoing breastfeeding can be considered without adverse health consequences for infants living in affluent countries. Even less evidence on the consequences of the timing of complementary food introduction is available for formula-fed infants.