Preheat oven to 350 degrees F. Put water in a large pot on high heat to boil. While waiting for the water to boil, brown ground beef and drain. Add spaghetti sauce & heat until warm.
When water is boiling, break spaghetti and add to water. Boil until tender. Drain spaghetti and place in a 9 x 13 pan. Pour sauce & ground beef mixture over spaghetti. Spread pepperoni on top of mixture. Spread cheese on top.
Bake in oven for 10 - 15 minutes, or until cheese in completely melted.