The relationships between biological indicators and exposure or tissue burdens are determined by the pharmacokinetic behaviour of the chemical. They can be studied by pharmacokinetic models of various types. Simple pharmacokinetic models are used here to describe general relationships valid for large groups of chemicals or situations. Important parameters to consider are the half-life of the biological indicator, the individual variability and the exposure variability. Biological sampling strategies are presented for monitoring of groups of workers, or individual workers. For specific chemicals, mainly solvents, more elaborate models can be developed, i.e., physiologically-based pharmacokinetic models including physiological, metabolic and physicochemical parameters. Such models are useful to describe the influence of confounding factors. Physiologically-based pharmacokinetic models can also be developed for metals and metalloids. Antimony is presented here as an example. In conclusion, pharmacokinetic modeling brings much information on sampling time, sample size, limit values, effect of physical workload and of individual physiological parameters.