A bio indicator is an indicator species or group of species whose function status can reveal the qualitative status of the environmental condition. For example, copepods and other small water crustaceans that are present in water can be checked for biochemical, physiological, or behavioral changes that may indicate a problem within their ecosystem. Bio indicators can tell us about the effects of pollutants in the ecosystem and about how long a problem may have been present, which chemical and physical testing cannot do. It can provide additional information about the amount and intensity of the exposure.

A biological indicator  assesses the sterility of an environment through the use of resistant microorganism strains (eg. Bacillus or Geobacillus).  As biological indicators use highly resistant microorganisms, you can be assured that any sterilization process that renders them inactive will have also killed off more common, weaker pathogens.