It was created by The Treaty of 9 June 1855 between the United States and members of the Walla, Cayuse, and Umatilla tribes.

[1] The largest community is Mission, which is the site of the tribal headquarters as well as the Umatilla Agency of the Bureau of Indian Affairs.

[4] The CTUIR is one of several tribal governments in the northwestern United States to offer free bus service on its reservation.

[5] The tribes developed the Wildhorse Casino Resort on their reservation to generate revenues for their people.

[6][7][8] The CTUIR founded the Tamástslikt Cultural Institute, a museum that provides both historical and contemporary exhibits related to the confederated tribes, as well as exhibits of contemporary Native American arts and traditional craftwork.