Zooperiphyton (e.g., the Protozoa) can be very useful indicators of water quality. Their ability to rapidly colonize artificial substrates provides an assessment capability not generally available for higher organisms. In addition, one can collect a sufficient quantity for statistically reliable results without endangering a local population through overcollecting, as is often the case for higher organisms. There are still other advantages in use of periphyton. However, no method or taxonomic group should be used automatically in every situation. Data redundancy, cost, and speed in gathering data are factors to be kept in mind; and the ability to measure a biological parameter does not mean that it should always be measured.