Bosses are stronger enemies that take longer to kill then other enemies. They can only be encountered on levels, and will always have other symbiotic helping them. Some bosses have a special ability they can use. Electro (both normal and symbiote) can heal himself, and Venom can make himself "invincible" ( If Spiderman touch's Venom, he will get pushed away by the symbiote [this is only during the chase scene]) to being harmed by Spiderman
.
This is a list of Bosses encountered in the game
Venom, Electro, Black Cat, Vulture, Wolverine, Symbiote Electro, Symbiote Black Cat, Symbiote Vulture, and Symbiote Wolverine.