The use of the word "artificial" in referring to introduced substrates in aquatic ecosystems has had some unfortunate consequences. The most serious is that many research investigators believe that the species that colonize these substrates are not characteristic of those that colonize natural substrates, and that the community structure and functional dynamics are quite unlike those that occur on natural substrates. Although the introduced substrates may have a chemical composition alien to natural environments (e.g., polyurethane), they are essentially inert, a characteristic common to many other natural substrates, such as rocks. The fact that they are free of aquatic organisms when introduced makes them no more alien to the ecosystem than a rock that has tumbled into a stream in a rockslide. Their location may be atypical (e.g., a substrate positioned in the epilimnion of a lake), but the species that have colonized the substrate after a week or so of exposure are those that inhabit the surface of solid substrates in the benthic area of the lake or in the shallow water along the shoreline. Most introduced substrates are chosen because they do not decompose and are not quickly eroded by various water actions. In short, they are quite inert and merely furnish a surface for attachment of both living and nonliving materials. Thus, although their composition may be artificial, the biological events that occur on them appear to be quite similar to those that occur on natural substrates of similar structural characteristics.