Biomarkers provide an early indication of the presence and/or effects of environmental contamination. These tools have the advantage of being able to characterize the mode(s) of action of a contaminant, with some biomarkers being relatively specific to a particular type of contaminant. Biomarkers can be used directly on organisms native to the environment of interest, as well as in bioassays.Acetylcholinesterase activity and feeding rate are two relevant biomarkers for measuring the effect of pollutant exposure in several groups of invertebrates and fish. However, further research is needed to limit the artefacts involved in interpreting their values and to obtain increasingly effective results.