A species can be listed in two ways. The United States Fish and Wildlife Service (FWS) or NOAA Fisheries (also called the National Marine Fisheries Service) can directly list a species through its candidate assessment program, or an individual or organizational petition may request that the FWS or NMFS list a species. A "species" under the act can be a true taxonomic species, a subspecies, or in the case of vertebrates, a "distinct population segment." The procedures are the same for both types except with the person/organization petition, there is a 90-day screening period.

How many different ways can a species be added to the endangered list?