The main methodological problems encountered in occupational epidemiology are related to the particularity of the risk factors concerned. It is necessary to account for the "Healthy Worker Effect" in the design, analysis and interpretation of surveys since the very presence of workers at particular jobs is related to their health. In cohort studies, it is necessary to have precise knowledge of the work environment corresponding to different jobs as exposure information is generally not individual, objective and continuously recorded. In case-control studies also, information on occupational exposures is a key issue, not only from the point of view of differential bias but also because exposure that is too loosely defined leads to dilution and reduced accuracy.