Employing a literature review approach, this article explores the evidence about missed opportunities for childhood immunizations, their impact on immunization rates, and potential solutions. It concludes that missed opportunities contribute significantly to the underimmunization of children. A concerted effort to reduce missed opportunities, particularly those that are relatively easy to eliminate, could yield measurable improvements in immunization levels of children. Missed opportunities for vaccinations have been found to occur in virtually all primary care settings, and their impact is greater for populations having low baseline immunization rates. Coverage levels could be improved by up to 30% in many settings by eliminating missed opportunities. Major reasons for missed opportunities include providers lacking knowledge about immunization status, provider or practice policies resulting in failure to vaccinate, and inappropriate contraindications. Solutions include promulgation and acceptance of useful guidelines for delivery of immunizations, provider and patient education, assessment and feedback about a practice's missed opportunities, and modification of routine practice styles within primary care and other health care settings.