Prevention of invasive infections caused by Haemophilus influenzae type b (Hib) bacteria has become possible with new vaccines. Covalent coupling to a protein carrier increases the immunogenicity of Hib capsular polysaccharide, and changes it to a T-cell dependent antigen. In clinical use, the conjugate vaccines are immunogenic in infancy, induce immunologic memory, and protect efficiently from invasive infections. They even seem to reduce oropharyngeal carriage of Hib bacteria. Wide-scale use of Hib conjugate vaccines has demonstrated their safety and low reactogenicity. Studies to combine them with other childhood vaccines are in progress. First signs of reduction of severe Hib infections can already be seen in countries where Hib conjugate vaccines are in wide use.