Fever is a frequent systemic adverse event following immunization, especially in infants and young children. Any fever after immunization may be caused by immunization or may coincide temporally as an indication of underlying disease, usually an infectious one. The time pattern of fever attributable to immunization has characteristic features depending on the vaccine used. Comparability of fever rates associated with different vaccines, or the same vaccines in different studies, is frequently hampered by the use of different definitions and/or assessment techniques for fever. A recent analysis by the Brighton Collaboration has provided a standardized case definition for fever, the use of which should be strongly encouraged.