Biological monitoring (BM) is a useful tool for exposure assessment of substances that can be absorbed through the skin or where control relies on respiratory protection. This commentary briefly describes the various types of biological monitoring guidance values (BMGVs) that are available to help interpret BM results. Population background, reference and biological equivalent values are available to help interpret BM data in a public health context and occupational guidance values are available for common workplace substances to help interpret results in terms of exposure rather than health. There is a clear need for occupational hygiene and health professionals to understand the basis of the guidance values and not use them as simple 'limits'. There is also a clear need for more peer-reviewed published studies of workplace exposure to help develop BMGVs.