0! = 1! = 1

The idea of the factorial (in simple terms) is used to 
compute the number of permutations of arranging a set 
of n numbers. It can be said that an empty set can only 
be ordered one way, so 0! = 1

That's informal.  Moe formally, we can do this:

By definition of factorial,

n! = n•(n-1)! where n > 0
1! = 1•(1-1)!
 1 = 1•0!
 1 = 0!

Edwin