Throughout the history of vaccination, vaccines have been accused of harmful side effects. Adverse events following immunization may be caused by the active antigen in the vaccine or other constituents, such as adjuvants, or may merely be coincidental. Possible neurological side effects have always obtained high attention. However, the risk of serious events caused by existing vaccines or aluminum adjuvants is very small. Currently, there are several new vaccines and adjuvants in the pipeline. Of these vaccines, many will be offered mainly to adolescents or adults. When taken into general use, some of them will probably be associated with serious adverse events. Although coincidence will be the most probable explanation in most cases, causality will have to be discussed in many situations. Preparing to address the causes of these adverse events is important.