Definitions of terms used in this report are given in Appendix B. We use the terms nonindigenous and indigenous to describe a species’ geographic range. These terms are always dependent on the scale of reference and geographic units used. The scale may be based on hydrologic units (e.g., watershed, drainage, basin), geopolitical boundaries (e.g., county, state, country), or some other geographical unit (e.g., continent, hemisphere). For example, the flathead catfish Pylodictis olivaris is indigenous to the central and south central United States and northeastern Mexico. Each of the following statements is true, depending on scale of reference: the flathead catfish is indigenous to northwestern Georgia; it is indigenous to the Mississippi River basin; and it is indigenous to North America. Flathead catfish have been released by humans into rivers of southeastern Georgia, and these introductions represent nonindigenous populations (i.e., the species is not indigenous to southeastern Georgia). We generally avoid using the terms exotic and transplanted, partially because of the misleading connotation that introduction of a species into an adjacent watershed poses a less serious threat than establishment of a foreign species in the same system.