The purpose of the Endangered Species Act is "to provide a means whereby the ecosystems upon which endangered and threatened species depend may be conserved, and to provide a program for the conservation of these species." The Act defines three fundamental terms as follows:

• Endangered means a species of fish, animal or plant is "in danger of extinction throughout all or a significant portion of its range". (For salmon and other vertebrate species, this may include subspecies and distinct population segments.)
• Threatened means a species "is likely to become endangered within the foreseeable future". Regulations for a threatened species may be less restrictive than if it were endangered; the difference is likely to be minor for Puget Sound Chinook salmon.
• Critical habitat means "specific geographical areas that are…essential for the conservation and management of a listed species, whether occupied by the species or not".

Section 4: Listing of a species
The National Marine Fisheries Service is responsible for listing Chinook salmon and other sea-going and marine species; the U.S. Fish and Wildlife Service is responsible for listing terrestrial and freshwater aquatic species. The agencies may initiate reviews for listings; citizens may also petition for them. A listing must be made "solely on the basis of the best scientific and commercial data available". After proposing a listing, agencies receive comment and conduct further scientific reviews for 12 to 18 months, after which they must decide if a listing is warranted. Economic impacts cannot be considered in this decision, but it may include an evaluation of the adequacy of local and state protections. Critical habitat for the species may be designated at the time of listing.

Section 7: Consultation
Even when a listing has only been proposed, all federal agencies must insure that any action they authorize, fund, or carry out is not likely to jeopardize the continued existence of a listed species nor adversely modify its critical habitat. This includes private and public actions that require a federal permit. Once a final listing is made, non-federal actions are subject to the same review, termed a "consultation". If the listing agency finds that an action will "take" a species (see Section 9 below), it must propose mitigations or "reasonable and prudent" alternatives to the action; if the proponent rejects these, the action cannot proceed.

Section 9: Prohibition of Take.
It is unlawful to "take" an endangered species, including killing or injuring it or modifying its habitat in such a way that interferes with essential behavioral patterns including breeding, feeding or sheltering.

Section 10: Permitted Take
Through voluntary agreements with the federal government that provide protections to an endangered species, a non-federal applicant may commit a take that would otherwise be prohibited as long as it is incidental to an otherwise lawful activity (such as developing land or building a road). A "Habitat Conservation Plan" (HCP) is the most likely such agreement that King County may pursue (see opposite side of this page).