This publication covers a wide range of topics connected with the concepts of complementary feeding. To start with, the focus is on breastfed infants as reference, looking at human milk as a model for the development of optimal foods in the complementary feeding period. The nutritional safety and quality of complementary foods, as well as the role of cereals, meat, dairy fermented products and local resources are discussed next, together with the possible implications for the use of functional nutrients. Within this context, the dietary requirements of children with certain medical conditions (from malnourished infants to those suffering from food allergy or celiac disease) are critically reassessed in the light of most recent observations. The presentations, coming from developing and transition to well-developed countries, remind us that the complementary feeding recommendations are still influenced by a mixture of tradition, history, policy, practice and science, and that any innovation should take into account all these points to be successful. This publication provides pediatricians, neonatologists, nutritionists and dieticians with a complete update on what is known and what is still unknown about the period when complementary feeding is implemented.