Streptococcus pneumoniae (S.pneumoniae) is a respiratory pathogen associated with high morbidity and mortality worldwide. Nasopharyngeal colonisation is necessary for person to person transmission. Over 90 pneumococcal serotypes have been identified and it is estimated that at any one time, 60% of children carry the bacterium asymptomatically. Through the use of mathematical modelling, this research aims to investigate the competitive effects amongst serotypes and to quantify the likelihood of serotype replacement as a result of the introduction of the pneumococcal conjugate vaccine in Scotland.