In collaboration with WHO, IMMUNIZATION basics analyzed 126 documents from the global grey literature to identify reasons why eligible children had incomplete or no vaccinations. The main reasons for under-vaccination were related to immunization services and to parental knowledge and attitudes. The most frequently cited factors were: access to services, health staff attitudes and practices, reliability of services, false contraindications, parents' practical knowledge of vaccination, fear of side effects, conflicting priorities and parental beliefs. Some family demographic characteristics were strong, but underlying, risk factors for under-vaccination. Studies must be well designed to capture a complete picture of the simultaneous causes of under-vaccination and to avoid biased results. Although the grey literature contains studies of varying quality, it includes many well-designed studies. Every immunization program should strive to provide quality services that are accessible, convenient, reliable, friendly, affordable and acceptable, and should solicit feedback from families and community leaders. Every program should monitor missed and under-vaccinated children and assess and address the causes. Although global reviews, such as this one, can play a useful role in identifying key questions for local study, local enquiry and follow-up remain essential.