The index of biotic integrity is a regionally based index used to measure the integrity of rivers and streams, and to determine the level of their biotic impairment. The IBI relies on multiple parameters (termed "metrics") based on fish community structure and function, to evaluate a complex biotic system. In order to implement biological criteria, a formal method for sampling the biota of streams, evaluating the resulting data, and clearly describing the condition of sampled stream reaches is needed. The IBI incorporates professional judgment with quantitative criteria that enables determination of a continuum between very poor and excellent conditions. An important key to successful restoration, mitigation and conservation efforts is having an objective way to assess and compare the biological integrity of damaged sites. The IBI provides a tool for doing so and, at the same time, allows managers to set specific biological integrity targets for restoration programs.