In occupational and environmental health we study the relationship between exposure to chemical, physical, biological, ergonomic and psychosocial factors at work or in the environment and the incidence of disease. Epidemiological and experimental in vivo and in vitro research generates a lot of studies which are used to decide whether a relationship is causal. Since our knowledge is mostly incomplete and imperfect and studies can point out in different directions, a set of criteria is used to decide on the causality of the association. A commonly used set of standards for distinguishing causal from noncausal associations has been advanced by Bradford Hill (1965): strength, consistency, specificity, temporality, biologic gradient, plausibility, coherence, experimental evidence and anology. For carcinogenic agents the criteria formulated by the International Agency for Research on Cancer are more popular. Some groups use Qualitative Bayes or Degree of Certainty of Causality to evaluate a relationship.