Group B streptococcus (GBS) or Streptococcus agalactiae is a Gram-positive, catalase-negative coccus. It is an opportunistic human pathogen that can cause disease in newborns, pregnant women and non-pregnant adults. Illness due to GBS can manifest as pneumonia, meningitis, septicaemia, and bone and/or soft tissue infections. The pathobiology of GBS infection indicates that the organism must be able to traverse a number of diverse cell types, including epithelial and endothelial cells, to cause disease in the host. To accomplish this, GBS possess an array of virulence factors to overcome the different barriers posed by host cell types. This review summarizes the known and proposed virulence factors of GBS, including capsule, lipoteichoic acid, CAMP factor, beta-haemolysin, superoxide dismutase, C5a peptidase, extracellular matrix protein interactions, hyluronate lyase, C-protein, two-component regulatory systems, and induction of apoptosis.