In this thesis I analysed population-based data on 22955 infants enrolled in a neonatal vitamin A supplementation trial in rural Ghana to investigate whether low birth weight (LBW: born weighing <2.50kg) was a risk factor for under-vaccination. I also investigated whether under-vaccination among LBW infants was occurring within a broader context of poorer health outcomes such as increased mortality, illness and health facility admissions and lower care-seeking. I additionally investigated how using routine contacts with health services (opportunities for vaccination) could be used to improve their vaccination. Compared to non-LBW (NLBW) infants, LBW infants were less likely to be vaccinated in both the neonatal and postneonatal period. The smaller the baby at delivery the less likely they were to be vaccinated (p-trend <0.0001). By the end of the neonatal period, moderately LBW (MLBW) infants (1.50-1.99kg) were 1.6 times (adjusted odds ratio (aOR)=1.64; 95%CI:1.30-2.08), and very LBW (VLBW) infants (<1.50kg) were 2.4 times (aOR=2.42; 95%CI:1.50-3.88) more likely to be BCG unvaccinated. In the postneonatal period, VLBW infants had an almost 40% lower DTP1 vaccination rate at age 10 weeks (adjusted rate ratio (aRR)=0.58; 95%CI:0.43-0.77) and 18 weeks (aRR=0.63; 95%CI:0.50- 0.80). MLBW infants had vaccination rates approximately 25% lower at these time points. Similar results were observed for DTP3. LBW infants had much higher mortality rates in infancy compared to NLBW infants. Infants weighing 2.00-2.50kg were >2 times (adjusted hazard ratio (aHR)=2.13; 95%CI:1.76-2.59); MLBW infants were >8 times (aHR=8.21; 95%CI:6.26-10.76), and VLBW infants were >25 times (aHR=25.38; 95%CI:18.36-35.10) more likely to die. The trend of higher mortality with lower birth weight was seen in each of the neonatal, early and late infant periods, but the magnitude of the association declined over time. There was also some evidence that LBW infants had increased illness rates in the neonatal period, and in each of the neonatal and early infant periods. An absence of care seeking was found for MLBW infants in the first year of life (aOR=1.46; 95%CI:1.18-1.81), and in each of the neonatal (aOR=3.30; 95%CI:1.98-5.48) and early infant periods (aOR=1.74; 95%CI:1.26-2.39) respectively. No association was found in the late infant period (p-interaction=0.0002). Among all infants (NLBW and LBW) with opportunities for vaccination, most opportunities were missed. There was no association between birth weight and uptake of opportunities. In conclusion LBW infants are under-served by vaccination in Ghana. Given their poorer health outcomes, efforts to improve their access to care services, including vaccination are warranted. Further research into the barriers and facilitators of vaccination of LBW infants is warranted, including qualitative research targeting care givers and vaccine providers.