As specific molecular target of organophosphate and carbammate pesticides, acetylcholinesterase activity and its inhibition has been early recognized to be a human biological marker of pesticide poisoning, but in the last two decades the interest for acetylcholinesterase has increased a lot in the ecotoxicological field; its determination has become a tool for monitoring environmental contamination and organismal exposure to those compounds in the biota. Furthermore its use in different phyla, either vertebrates or invertebrates makes acetylcholinesterase a particularly versatile biomarker that can be used to investigate pollutant effects at many trophic levels and in different environments.