A quantum field theory defined on an orbifold becomes singular near the fixed points of "G". However string theory requires us to add new parts of the closed string Hilbert space — namely the twisted sectors where the fields defined on the closed strings are periodic up to an action from "G". Orbifolding is therefore a general procedure of string theory to derive a new string theory from an old string theory in which the elements of "G" have been identified with the identity. Such a procedure reduces the number of states because the states must be invariant under "G", but it also increases the number of states because of the extra twisted sectors. The result is usually a perfectly smooth, new string theory.