Group B Streptococcus (GBS) is a common cause of sepsis and meningitis in newborns, and causes disease in pregnant women and non-pregnant adults. The incidence of disease among non-pregnant adults, particularly those with underlying conditions, is increasing. In addition, many individuals are asymptomatically colonized with GBS. When compared to group A Streptococcus and Streptococcus pneumoniae, however, little is known about the pathogenesis, natural history and transmission dynamics of GBS. Various molecular tools have been utilized to study this organism, including both phenotypic techniques, such as serotyping and multilocus enzyme electrophoresis, and genotypic techniques such as plasmid analysis and pulsed-field gel electrophoresis. This review outlines the contributions of these methods to our current understanding of GBS infections.