The literature since 1985 on the biological monitoring of occupational, environmental, or iatrogenic exposures of humans to nickel is surveyed from the author's perspective. Urine and serum are the body fluids commonly analyzed for nickel. Nickel concentrations in urine specimens from unexposed persons usually exceed the current analytical detection limits, while nickel concentrations in their serum specimens are close to the detection limits. For this reason, and because urine collection is painless, noninvasive, and convenient, urine is more practical than serum for the biological monitoring of nickel-exposed workers. Electrothermal atomic absorption spectrometry with Zeeman background correction is currently the method of choice for the analysis of nickel in biological materials. Procedures for quality assurance and strategies for biological monitoring are summarized, as performed in the author's laboratory, and current reference values are listed for nickel concentrations in urine and serum specimens from healthy unexposed persons.