Place the tomatillos and peppers in a soup pot and cover with 6 cups of water. Bring to a boil then reduce heat to a simmer. Simmer, uncovered, for 12-15 minutes, being careful not to let the tomatillos burst.
Drain pot and reserve ½ cup of the cooking liquid. In a blender, puree the cooked tomatillos and peppers along with the garlic and reserved cooking liquid. Season with salt and pepper to taste.