The Environmental Protection Agency, commonly abbreviated to EPA, is an organization that is or was a part of the United States Armed Forces that was headed by the evil mastermind Russ Cargill. Its job was to "protect" the environment by using brute force with the EPA army. They are the tertiary antagonists of The Simpsons Movie (the main being Russ Cargill and the secondary being Charles Montgomery Burns).
The EPA caught Mr. Burns illegally dumping radioactive waste in the city park and fined him three million dollars. The agency also stopped Homer from killing the Screamapillar and made him and Marge take care of it due to it being an endangered species .
Later, under the command of Russ Cargill, the EPA was primarily responsible for tricking President Arnold Schwarzenegger in encasing Springfield in a dome that later became known as Trappuccino. This action was taken in response to severe environmental degradation of the Springfield area in general, and Lake Springfield in particular. The EPA had also caught a mutant Multi-eyed squirrel in Springfield, and attributed its existence to environmental conditions in the town. After their plan to destroy Springfield failed, it was presumed the organization might have been exposed to the whole world, disbanded by the President and they've been arrested for their actions.
EPA agents generally use helicopters to get around.