Bacterial conjugate vaccines are used in infants, adolescents, and the elderly, and they are among the safest and most successful vaccines developed during the last 40 years. Conjugation of polysaccharides to proteins provides T cell epitopes that are necessary in the germinal centers for the affinity maturation of polysaccharide-specific B cells. Collective analysis of data from animal experiments and clinical trials, reviewed with current knowledge of immunology, revealed possible mechanistic explanations that may improve our understanding of conjugate vaccines. Key conclusions are that naïve infants respond differently from adolescents and adults and that most of recommended schedules generate only 10 to 35% of the maximal antibody titer that the vaccine can induce, indicating that the full potential of glycoconjugate vaccines has not yet been reached.