This is a book about the regulation of chemical risks; this chapter specifically concerns the regulation of chemicals in the occupational setting. People are exposed to a variety of chemicals during their life; some are to our knowledge not harmful while others are. Working life may be a major contributor to a person's accumulated chemical exposure. A number of diseases have been related to the occurrence of harmful- substances in the occupational setting, for instance asthma, allergies and several forms of cancer. One can conclude that the risks associated with chemicals exposure and their regulation in the work place is well worth scientific scrutiny. Occupational exposure limits (OELs) are limits of concentrations of specific substances in the air, averaged over a period of time. The rationale behind OELs is that if the dosage of a chemical is -sufficiently low, no or acceptably low adverse health effects will arise. The dose-response relationship differs of course with the different inherent traits of the specific chemical. For some chemicals evidence suggests that a negative health effect only occurs above a certain level of exposure, this means that a safe level exposure is possible- to achieve. For many chemicals this is not the case though, either there is not enough knowledge to derive a no effect level (NOAEL), if such one does indeed exist, or there is in fact a linear dose-response relationship without any threshold. In the -latter case low-level exposure might only lead to very low individual risks but if many -persons are exposed the collective exposure result in substantial population effects.