Haemophilus influenzae is a small gram-negative pleomorphic coccobacillus. The H influenzae type b (Hib) strain is especially important because, prior to the advent of effective Hib vaccines, the organism was the cause of 95% to 98% of severe invasive infection, including meningitis, epiglottitis, cellutitis, sepsis, pneumonia, and osteomyelitis, in children less than 5 years of age. An important microbiologic feature of H influenzae is its development of resistance to a wide variety of antibiotics, including sulfonamides, trimethoprim-sulfamethoxazole, erythromycin, tetracycline, and penicillin. Ampicillin resistance is now widespread, ranging between 5% and 40% of all isolates in various parts of the world. The current mainstays of treatment are third-generation cephalosporins, but the potential for increased resistance to them, as well as other factors, has underscored the need for prophylactic measures. Since the early 1970s, when the capsular polysaccharide of Hib was purified, characterized, and shown to be immunogenic, tremendous strides have been made worldwide to immunize children against the organism and to irradicate the bacteria. A steady decline in the incidence of Hib diseases has occurred since the introduction in 1990 of Hib conjugate vaccines for infants. Because of this rapid decline, Hib diseases among children less than 5 years of age is targeted for elimination in the United States. This article provides an overview of the discovery and characterization of the organism and a review of the literature on the impact that the Hib vaccines have had on morbidity and mortality rates among children less than 5 years of age.