Running is the final leg of a traditional triathlon. In the standard, or Olympic, format, the running section is 10 kilometers (6.2 miles) long. Since running involves less specialized equipment or controversial clothing than cycling and swimming, there's less rulebook space devoted to it, but you can still be disqualified or penalized for "running foul" of triathlon law.
You can walk during the running portion of a triathlon, but please don't "crawl." In typical legalese style, the ITU defines crawling as "the action by an athlete of bringing three or more limbs in contact with the ground, either together or in any sequence, to enable forward propulsion" [source: ITU]. The rules don't explicitly mention piggyback rides, but those are probably covered in the clause forbidding "unauthorized assistance."
Since the running portion comes directly after the biking section, don't forget to take off your helmet. Running with a bike helmet is grounds for immediate disqualification. So is running without a shirt. And even though barefoot running is all the rage, triathletes must wear shoes at all times during the running portion and must also leave their iPods and cell phones at home. Breaking any of these rules can lead to disqualification.
Since running is the final portion of the triathlon, there are some very specific rules regarding the race's finish. Here again, the rule makers get to use their favorite word: torso. According to the ITU, a runner is said to have "finished" the race when "any part of the torso reaches the perpendicular line extending from the leading edge of the finish line" [source: ITU]. And in case you were unsure about what a "torso" is, exactly, the ITU defines it as any part of the body above the waist "distinct from the head, neck, arms, legs, hands or feet" [source: ITU]. That explains why runners always break through the finish line ribbon with their chests.
For lots more great information about triathlons, marathons and other endurance sports, run over to the links on the next page.