Periphyton are very useful in developing biocriteria for streams. The metrics currently used by States for periphyton are well developed and are used to determine if a stream is supporting aquatic life use as designated under the Clean Water Act. Because periphyton are the major primary producers in most stream systems and are often the base of the food web, they are important indicators of biological integrity. Periphyton have been used for over 50 years for monitoring streams and rivers and several reports are available that describe the ecological characteristics of the taxa in these assemblages. In addition, there are several databases that describe the known ranges of taxa along chemical, physical, and pollution gradients. Analysis of variance components and historical assessments of stream and river quality are possible with the many samples that have been collected and archived in museums during the last 90 years.