Toxicity measurement devices measure general toxicity to biological organisms, and detection of toxicity in any water/wastewater asset can indicate a potential threat, either to the treatment process (in the case of influent toxicity), to human health (in the case of finished drinking water toxicity) or to the environment (in the case of effluent toxicity). Currently, whole effluent toxicity tests (WET tests), in which effluent samples are tested against test organisms, are required of many National Pollutant Discharge Elimination System (NPDES) discharge permits. The WET tests are used as a complement to the effluent limits on physical and chemical parameters to assess the overall effects of the discharge on living organisms or aquatic biota. Toxicity tests may also be used to monitor wastewater influent streams for potential hazardous contamination, such as organic heavy metals (arsenic, mercury, lead, chromium and copper) that might upset the treatment process.

The ability to get feedback on sample toxicity from short-term toxicity tests or toxicity "meters" can be valuable in estimating the overall toxicity of a sample. On-line real time toxicity monitoring is still under active research and development. However, there are several portable toxicity measurement devices commercially available. They can generally be divided into categories based on the different ways they measure toxicity: