Temporary assembly centers were the first stop for most internees. Sixteen centers were established in California, Oregon, Washington State, and Arizona. Fairgrounds, racetracks, and other public facilities were pressed into service to handle the influx of Japanese Americans. Internees remained in these centers, under the control of the Army's Wartime Civil Control Administration (WCCA), until the War Relocation Authority (WRA) camps were ready.

Conditions in the WCCA assembly centers were unsanitary at best. At Tanforan and Santa Anita, California, internees were housed in stalls that only a week earlier had held horses. Sanitation, food service, and health-care facilities were beneath the lowest U.S. Army standards. Many internees had to stay in assembly centers for several months before being moved to permanent camps.

"The stall was about ten by twenty feet and empty except for three folded Army cots lying on the floor. Dust, dirt, and wood shavings covered the linoleum that had been laid over manure-covered boards, the smell of horses hung in the air, and the whitened corpses of many insects still clung to the hastily white-washed walls." — Yoshiko Uchida, Desert Exile: The Uprooting of a Japanese American Family

"On warm days it was unbearable in the stalls and barracks. The stench of manure returned with the heat, and this in turn brought back the horseflies. Most of the people remained outdoors on such days, and usually I did too, but there were times when I kept working inside. Later, by the order of the medical authority, all windows in the stalls were hinged so that they could be opened." — Mine Okubo, Citizen 13660