Tsukiji is the largest fish market on earth. Every day over 2,000 tonnes of fish are sold to buyers from all over Tokyo, whether they be department stores or gourmet sushi chefs. If you can't bear (or just can't) get there early enough to get tickets for the daily auction (starts at 5:25am), then you can still enjoy the retail area, vegetable market, and intermediate wholesalers fish market. Just watch out for buyers, sellers, and those lethal turret trucks...