All vaccines cause some adverse events; serious adverse events are rare. Causal associations between a vaccine and an adverse event rarely can be determined by specific tests such as identifying a vaccine agent in the affected tissue of patients. In the absence of such data, epidemiologic studies can be used to determine if the risk of the disorder is increased in vaccinated compared to unvaccinated individuals. Common mistakes include assuming a causal relationship based on a temporal association only or a series of affected patients. Careful studies have demonstrated that many hypothesized causal associations between vaccines and adverse events were not substantiated. False assumptions regarding causality are likely to occur for illnesses without a carefully defined etiology or pathogenesis.