Sumo is a competitive contact sport where a wrestler attempts to force another wrestler out of a circular ring or to touch the ground with anything other than the soles of the feet. The sport originated in Japan, the only country where it is practiced professionally. The Japanese consider sumo as a modern Japanese martial art, though the sport has a history spanning many centuries. The sumo tradition is very ancient, and even today the sport includes many ritual elements, such as the use of salt for purification, from the days sumo was used in the Shinto religion. Life as a sumo is quite difficult, with rules laid down by the Sumo Association. Professional sumo wrestlers are required to live in communal "sumo training stable”.

Professional sumo is organized by the Japan Sumo Association. The members of the association, called oyakata, are all former wrestlers, and are the only people entitled to train new wrestlers. All practising wrestlers are members of a training stable run by one of the oyakata, who is the stablemaster for the wrestlers under him. Currently there are 54 training stables for about 700 wrestlers.

Sumo wrestling is a strict hierarchy based on sporting merit. The wrestlers are ranked according to a system that dates back hundreds of years, to the Edo period. Wrestlers are promoted or demoted according to their previous performance, and a carefully prepared banzuke listing the full hierarchy is published two weeks prior to each sumo tournament.

The biggest sumo wrestler is a 44 year old man, Emanuel Yarbrough, who wieights over 880 lbs (400 kg).