Nickel is an important metal in the automobile industry, in electronics, as a catalyst in chemical processes, in nickel-cadmium batteries and accumulators, in many household products, and in cheap jewelry. Almost everyone in the industrially developed countries may be in daily contact with nickel. Cutaneous nickel allergy (contact dermatitis) is very common, as typically 15% to 20% of the population have positive results in epicutaneous testing. Nickel sensitization may be avoided by restricting contact with objects that release nickel ions through sweat on skin. Because nickel is also carcinogenic to man, causing upper respiratory tract and lung malignancies, advanced control of exposure at workplaces is necessary. Control can be accomplished either by measuring the exposure in the occupational environment or through urinary nickel analysis by applying so-called biological monitoring. As covalent nickel adducts have not been found in DNA, the carcinogenic effect of nickel is probably related to its lipid-peroxidation properties, which induce DNA-strand gaps and breaks and DNA-protein crosslinks. The negative effect of nickel ions on glycoprotein metabolism may explain the nephrotoxic effects of excessive exposure.