After Atari announced its intent to enter the home computer market in December 1978, the Atari 400 and Atari 800 were presented at the Winter CES in January 1979 and shipped in November 1979.

The names originally referred to the amount of memory: 4 KB RAM in the 400 and 8 KB in the 800. By the time they were released, RAM prices had started to fall, so the machines were both released with 8 KB, using 4kx1 DRAMs. The user-installable RAM modules in the 800 initially had plastic casings but this caused overheating issues, so the casings were removed. Later, the expansion cover was held down with screws instead of the easier-to-open plastic latches. The computers eventually shipped with maxed-out RAM: 16k and 48k, respectively, using 16kx1 DRAMs.