More than 100 people attended the AIHce 2005 roundtable on government agency efforts in biological monitoring. The roundtable was organized to present information on the state of government human biological monitoring programs in the past, present and future. Biomonitoring is the general term to describe using biomarkers (chemical or physical markers) in all living things to estimate exposure, effect of exposure or the susceptibility to exposure, or the biomarkers left in the environment through contact between living things and their environments. Human biomonitoring focuses on the measurement of these biomarkers in people who are exposed at their worksites and during their everyday activities. Biological monitoring of workers is defined as the measurement of chemical markers of exposure to physical, chemical and biological agents. Biological monitoring reflects exposure from all possible routes, and is especially useful when dermal and oral exposures are present rather than, and in addition to, inhalation exposure, and when personal protective equipment is worn. Biomarkers, therefore, can range from measuring a chemical, its metabolite(s) or participating enzymes in body fluids (chemical markers) to identifying specific genes that may affect the outcome of an exposure (genetic markers). Benefits from biomonitoring include assessing current exposure, linking exposure and disease, identifying unknown or unsuspected exposures like dermal or oral exposures, following trends of exposure over time and evaluating the mechanism of action. However, drawbacks exist as well. Taking a biological sample may be invasive; biological monitoring may not be as useful if the toxic reaction is at the point of contact; little new information may be gleaned depending on the route of exposure and there may be low cost benefit. Biological measurements tend to have greater variability than other tools used in exposure assessment due to confounding variables like non-workplace exposures, diet, genetic makeup, previous exposures at work and lifestyle factors. Some workers fear that markers for drugs of abuse also may be measured in their samples in addition to the specific markers produced after exposure to specific agents. Employers may not favor biological monitoring because of the added expense relative to air monitoring. Sufficient information on the biomarkers, their relationships to the exposing agent and the other potential interferences in the specific environment involved must be available to allow biological monitoring data to be interpreted correctly; hence, the necessity to have documentation (for example, for the Biological Exposure Indices or for Biological Tolerance Values) to allow a thorough understanding of the science involved. Biomonitoring is only as good as the biomarker selected. Many factors affect the selection of the biomarker(s) for use in occupational health research or practice. The timing of the collection of the sample, the route of exposure, other exposures, the chemical properties of the agent and toxicity of the agent all affect what type of biomarker would provide the most useful information.