With the fluctuation of international oil prices due to limited conventional petroleum resources, unconventional energy, including bitumen and extra heavy oil, are of primary interest these days. However, in-situ recovery processes of these resources are somewhat complicated, and there are many technical challenges accompanying it. For the recovery process, it is widely known that thermal techniques using steam are essential in reservoir heating. Especially, the steam-assisted gravity drainage process is a leading technology in the development of oil sands. This article presents the steam-assisted gravity drainage process for the production of oil sands by reviewing and analyzing several authors and companies.