Rajadamnern Stadium is Bangkok’s oldest and most venerable venue for moo·ay tai (Thai boxing; also spelt muay Thai). Be sure to buy tickets from the official ticket counter or online, not from touts and scalpers who hang around outside the entrance.
Children under 120cm are allowed free entry (fights often get bloody, so it's probably best to not bring young kids along). There's a souvenir shop to the right of the ticket counters, where you can buy sundry accessories and paraphernalia associated with the sport, such as gloves and colourful shorts worn by contestants. The store is open during bouts.