Much interest has surrounded the use of conjugate vaccines in recent years, with the development of vaccines against disease caused by Haemophilus influenzae type b, Neisseria meningitidis, and Streptococcus pneumoniae. These vaccines offer the potential for safe and effective disease control, but some questions remain, particularly regarding the duration and mechanisms of protection and the longer-term impact of vaccination on carriage. In this paper, the authors use data on immunization with serogroup C meningococcal conjugate vaccines in England and Wales to develop and apply a mathematical model to investigate the direct and indirect (herd immunity) effects of a conjugate vaccine program. A realistic, age-structured, dynamic model was developed and parameterized and was fitted to epidemiologic data from England and Wales. The effects of a range of vaccine strategies, including hypothetical scenarios, were investigated. The basic reproduction number was estimated to be 1.36. Catch-up vaccination targeting teenagers generated substantial herd immunity and was important in controlling disease rapidly. The results were sensitive to changes in the assumptions regarding the method of vaccine action, particularly duration of protection and efficacy of vaccination against carriage acquisition. This model can be used to help predict the potential impact of vaccine strategies both in the United Kingdom and elsewhere.