The Monster World is the place where the Hormone Monsters and other monsters live and work.
The Monster World is first shown in The Department of Puberty when Andrew Glouberman, Jessi Glaser and Nick Birch find an open portal to the Monster World in Nick's closet after his hormone monster, Tyler, forgot to close it. The three kids go through it and find out that it's the location where the monsters work and live. They go to the Department of Puberty to find Nick a new hormone monster, where they meet Gavin, Ricky and other hormone monsters as potential candidates, but Maurice tells them that they can't stay in the Monster World for too long because if they do, they'll end up stuck in puberty for the rest of their lives. Andrew also meets the Shame Wizard once again, this time in his office. Jessi Glaser eavesdrops on what she finds out to be her assigned puberty monsters discussing her future, including the Depression Kitty, and upon meeting her decides to go with her to Depression Ward, a padded cell-like room, instead of following Connie. The Kitty doesn't want to let her escape but at the end the others help her out and they return to the normal world.