This chapter focuses on the deteminants of a number of immunization programme outcomes in Flanders (Belgium), such as vaccine initiation and uptake; completion of the vaccination schedule and compliance to official validity criteria. These were assessed in both infant and adolescent age groups. Three main groups of potential influencing factors are looked at: (1)individual background variables; (2)family level variables; (3)external factors such as the governmental vaccination programme and other initiatives to promote vaccination. Data on parental willingness to pay for and willingness to accept multiple concomitant injections and their determinants are discussed as well. Exploring relationships between vaccination programme outcomes and influencing factors can give important information to optimize vaccination programme performance.