With the successful implementation of childhood immunization programs in the United States, an increasing percentage of vaccine-preventable infections now occur in adults. By providing primary care services to adult women, midwives are in a unique position to halt the spread of these infections. Immunizations are often avoided in pregnancy and the early postpartum period, however, in the mistaken belief that vaccines are harmful to the fetus or neonate. This article, the first in a two-part series on immunizations, reviews the current epidemiology of vaccine-preventable diseases, discusses the indications and precautions for vaccine usage in pregnancy and the early postpartum period, and presents the current recommendations from the American Committee on Immunization Practices for the most commonly administered adult immunizations: tetanus-diphtheria, hepatitis B, influenza, pneumococcal, measles, mumps, rubella, and varicella. © 1999 by the American College of Nurse-Midwives.