On June 13, 2014, the California Department of Public Health (CDPH) declared that a pertussis epidemic was occurring in the state when reported incidence was more than five times greater than baseline levels. The incidence of pertussis in the United States is cyclical, with peaks every 3-5 years, as the number of susceptible persons in the population increases. The last pertussis epidemic in California occurred in 2010, when approximately 9,000 cases were reported, including 808 hospitalizations and 10 infant deaths, for a statewide incidence of 24.6 cases per 100,000 population. During January 1-November 26, 2014, a total of 9,935 cases of pertussis with onset in 2014 were reported to CDPH, for a statewide incidence of 26.0 cases per 100,000. CDPH is working closely with local health departments to prioritize public health activities, with the primary goal of preventing severe cases of pertussis, which typically occurs in infants. All prenatal care providers are being encouraged to provide tetanus, diphtheria, and acellular pertussis vaccine (Tdap) to pregnant women during each pregnancy, ideally at 27-36 weeks' gestation, as is recommended by the Advisory Committee on Immunization Practices (ACIP), or refer patients to an alternative provider, such as a pharmacy or local public health department, to receive Tdap.