The Terminal 5 area has been dedicated to marine cargo and maritime industrial use for more than 100 years.  In 1994, a combined U.S. Army Corps of Engineers, Washington Department of Ecology, and Port of Seattle federal-state Environmental Impact Statement (EIS) was prepared to comply with environmental review requirements prior to the 1999 cleanup, redevelopment, and expansion of the terminal to accommodate additional container services.

Dramatic changes in the container cargo transshipment industry occurred subsequent to the 1999 Terminal 5 facility redevelopment, and the pace of that change has accelerated in the last five years. Ultra-large container ships with capacity of more than 10,000 twenty-foot equivalent units (TEU) are now entering service at West Coast cargo terminals, with vessels of 18,000-TEU capacity expected in the coming years. These larger container ships would make fewer calls and only call at ports capable of accommodating them.

In response to industry changes, the Northwest Seaport Alliance (NWSA) proposes improvements at Terminal 5 to accommodate larger vessels. 

The Terminal 5 Improvements Project is a Port of Seattle and NWSA project. Acting as Lead Agency for the environmental review of the proposal, the Port of Seattle has hired a team of technical analysts to provide an objective analysis of the proposal and its likely impacts.