Chemical allergy describes adverse health effects that result from the stimulation of specific immune responses by chemicals. Hypersensitivity reactions are the result of normally beneficial immune responses acting inappropriately against benign antigens, causing inflammatory reactions and tissue damage. The two most frequent manifestations of chemical-induced allergy are contact hypersensitivity and respiratory sensitization, both of which can have serious impact on quality of life, and represent a common occupational health problems. Chemical agents cause approximately 30% of cases of occupational asthma and roughly 90% of these cases involve immunological mechanisms (allergy). Over the past few decades industrialized countries have witnessed a significant increase (although the rate of increase has recently slowed) in the prevalence of atopic diseases including atopic rhinitis, bronchial asthma and urticaria. Many factors, both intrinsic and extrinsic, can contribute to the development of chemical allergy. In particular, the immune response can be affected by the genetic background, pathological conditions, hormonal and central nervous system status, etc. and by chemical related factors, e.g. dose level, frequency, route and duration of exposure, biotransformation, pharmacokinetics, chemical reactivity, etc. Furthermore, chemical pollution, indoor environment, diet, vaccination programs and the "hygiene hypothesis" have also been implicated in the increased prevalence of asthma and atopic diseases. Following the influence of condition of exposure, heritable and acquired factors, such as age, genetic background, gender etc. on chemical allergy is discussed.