The highest infection rates with influenza virus occur in young children because of their lack of prior immunity and prior exposure to the virus. Most children with influenza virus infection are healthy otherwise. Previously healthy children who have influenza in their first year of life have a substantial risk for developing serious disease. An underlying chronic medical condition further increases this risk. Infants, young children, and children with underlying conditions have the highest rates of outpatient medical visits and hospitalizations for acute respiratory disease during influenza epidemics. Secondary bacterial infections, antibiotic use, and other complications of influenza are consequences of influenza virus infection in children of all ages. Annual immunization with influenza vaccine is recommended for any child older than 6 months of age in whom prevention of disease is desirable, and particularly in those with underlying medical conditions. The consequences of infection in infants younger than 6 months of age can be modified by maternal immunization currently recommended for women in the second or third trimester of pregnancy during the influenza season. Family members, including siblings, and all other close contacts should receive influenza vaccine to reduce the possibility of transmission to children at risk. Immunization of all children has a positive impact on the occurrence of influenza infection and its complications, both in those at greater risk and in the entire population.