Bacteria are remarkably adaptable organisms that are able to survive and multiply in diverse and sometimes hostile environments. Adaptability is determined by the complement of genetic information available to an organism and by the mechanisms that control gene expression. In general, gene products conferring a growth or survival advantage in a particular situation are expressed, while unnecessary or deleterious functions are not. Expression of virulence gene products that allow pathogenic bacteria to multiply on and within host cells and tissues are no exception to this rule. Being of little or no use to the bacterium except during specific stages of the infectious cycle, these accessory factors are nearly always subject to tight and coordinate regulation. As a result of recent advances, we are beginning to appreciate the complexities of the interactions between bacteria and their hosts. The ability to probe virulence gene regulation in vivo has broadened our perspectives on pathogenesis.