Why Moses and Elijah?

Matthew tells us, "After six days Jesus took with Him Peter, James and John the brother of James, and led them up a high mountain by themselves. There He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. Just then there appeared before them Moses and Elijah, talking with Jesus." Moses and Elijah? Why did this cameo appearance take place, and of all of the famous people of the Old Testament, why was it these two important historical figures?