Carlo Agostoni, MD, and Kwang Sik Kim, MD, are the Guest Editors for this annual review issue on Nutrition and the Microbiome. Dr Agostoni is a Professor of Pediatrics at the Department of Clinical Sciences and Community Health, University of Milan, within the Fondazione IRCCS Cà Granda Ospedale Maggiore Policlinico, Milan. Dr Kim is Professor of Pediatrics and Molecular Microbiology and Immunology, Johns Hopkins University School of Medicine and Bloomberg School of Public Health; and Director, Division of Pediatric Infectious Diseases, Johns Hopkins Children's Center, Baltimore, MD.