Despite extensive immunization, the disease pertussis remains one of the world's leading causes of vaccine-preventable deaths. An estimated 50 million cases and 300,000 deaths occur every year. A resurgence of pertussis is observed in highly immunized populations. Increasing numbers of pertussis are reported in adolescents and adults who transmit bacteria to newborns and infants to whom pertussis may be a life-threatening disease. Many studies have shown that the causes for the resurgence are multiple, such as increased awareness of disease, use of better diagnostic tools, improved surveillance methods and waning vaccine-induced immunity. Recently, antigenic divergence has been found between vaccine strains and clinical isolates in many countries with high vaccination coverage. Here, we summarize these findings and discuss the factors contributing to pertussis resurgence in immunized populations.