The chemical agents that are of importance in occupational hygiene are chemical elements or compounds; pure or in mixtures. They occur in nature or are the result of, they are being used or being released (also in the form of waste) during a professional activity. They are (intentionally) produced and possibly marketed. In many industries, workers come into contact with chemicals: the chemical industry, metal processing, woodworking, graphical industry, plastic industry,... But also detergents, paints, inks and solvents belong to this category. Exposure to chemical agents can lead to various health complaints : irritations of the respiratory tract and mucous membranes, irritation of the skin, concentration loss or even permanent health damage. In the following sections, specific information can be found regarding the toxicity of substances, specific concentration limits for exposure to these substances, legislation on these substances, measurement or calculation of exposure concentrations of these substances and corresponding data processing. We conclude with an item on generic risk analysis systems.