For the purposes of the paper, biocriteria are defined as numerical values that describe the biological health of aquatic communities for a designated aquatic life use. Regardless of whether they are implemented regionally or site specifically, biocriteria (ambient, community-based) have the following advantages compared to chemical and toxicological criteria: biocriteria facilitate direct assessment of designated aquatic life uses, are applicable to a wide range of stressors (physical, chemical, biological, point, nonpoint, toxic, nontoxic), and provide a firm basis for regulating nonchemical degradation and antidegradation. They offer a mechanism for evaluating long-term effects of spills, process changes, and illegal dumping; for assessing cumulative effects of multiple dischargers and stressors; and for estimating bioaccumulation and indirect effects (food chain, competition, predation, migration, life history).