Evidence-based guidelines for immunization of infants, children, adolescents, and adults have been prepared by an Expert Panel of the Infectious Diseases Society of America (IDSA). These updated guidelines replace the previous immunization guidelines published in 2002. These guidelines are prepared for health care professionals who care for either immunocompetent or immunocompromised people of all ages. Since 2002, the capacity to prevent more infectious diseases has increased markedly for several reasons: new vaccines have been licensed (human papillomavirus vaccine; live, attenuated influenza vaccine; meningococcal conjugate vaccine; rotavirus vaccine; tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis [Tdap] vaccine; and zoster vaccine), new combination vaccines have become available (measles, mumps, rubella and varicella vaccine; tetanus, diphtheria, and pertussis and inactivated polio vaccine; and tetanus, diphtheria, and pertussis and inactivated polio/Haemophilus influenzae type b vaccine), hepatitis A vaccines are now recommended universally for young children, influenza vaccines are recommended annually for all children aged 6 months through 18 years and for adults aged > or = 50 years, and a second dose of varicella vaccine has been added to the routine childhood and adolescent immunization schedule. Many of these changes have resulted in expansion of the adolescent and adult immunization schedules. In addition, increased emphasis has been placed on removing barriers to immunization, eliminating racial/ethnic disparities, addressing vaccine safety issues, financing recommended vaccines, and immunizing specific groups, including health care providers, immunocompromised people, pregnant women, international travelers, and internationally adopted children. This document includes 46 standards that, if followed, should lead to optimal disease prevention through vaccination in multiple population groups while maintaining high levels of safety.