Preheat oven to 375 degrees.
Remove the stems from all of the mushrooms and place the caps in an oven safe baking dish, face up. You may want to scrape out some of the insides of the mushrooms depending on their size, and how much sauce you want to fit.
Place one pepperoni at the bottom of each cap (I cut the pepperonis in quarters and place 4 quarters at the bottom of each mushroom so that they would fit better).
Spoon on pizza sauce until it just reaches the top. Sprinkle mozzarella cheese on top of each.
Bake mushrooms for about 25-30 minutes, or until cheese is golden brown.
