The Hulk is a raw embodiment of anger, power and might, a combination that makes him one of the most formidable characters in the Marvel Universe.
Bruce Banner's irate alter ego has been known to smash villains, heroes and even a city or two into next week, but he is far from invulnerable.
Every superhero has their own personal version of Kryptonite. With Superman, it is quite literally Kryptonite, while Green Lantern's is rather pathetically the colour yellow and Martian Manhunter doesn't take too well to fire.
In Hulk's case it's harder to pinpoint, but the jade giant has proven vulnerable to a number of things over the years, from a certain indestructible metal to cosmic powers.
His primary weakness, however, is that there's a meek and well-meaning scientist named Bruce residing at his core, and opponents who exploit this often get one over on Thor's friend from work or are able to manipulate him.
That said, very few beings from the Marvel Universe are capable of standing up to the green Goliath in a fist fight, so it comes as little surprise that the few who have defeated him either have incredible powers, got creative or were really flukey.