EPA's Region 9 office works to protect public health and the environment in the southwestern United States (Arizona, California, Nevada, and Hawaii). EPA Region 9 also works with 147 federally recognized tribes in the Pacific Southwest.

Additionally, the regional office's environmental responsibilities extend into the Pacific Ocean well beyond Hawaii. To the west and south of Hawaii are hundreds of islands with an estimated population greater than 300,000, where EPA has an active and direct role. The islands are scattered over an area larger than the 48 contiguous United States. These island jurisdictions consist of the U.S. territories of Guam and American Samoa, the Commonwealth of the Northern Mariana Islands, and other unincorporated U.S. Pacific possessions.