Fish are most frequently used as the so-called target organisms in developing criteria and setting standards for water quality. That is, the criteria for acceptable water quality, except for eutrophication, are usually based on fish, especially if they have economic or recreational value. Specific standards exist for dissolved oxygen (DO) and temperature that have been experimentally determined with fish and will be discussed in detail. The standards for toxicants are usually undefined, however, and the limits are determined by employing bioassays, either in situ or in vitro with the relevant water and species. That is due to variability in the effect of a given toxicant from water-towater and species-to-species. Nevertheless, published criteria for concentrations of particular toxicants are often used in the absence of onsite bioassay results to evaluate toxicity.