We consider differential forms invariant under a finite reflection group acting on a vector space V over a field of characteristic zero. Certainly taking exterior derivatives of invariant polynomials produces invariant differential forms, but does this account for all of them? In 1963, Solomon showed in fact that the algebra of invariant differential forms is freely generated by the first exterior derivatives of basic invariants. In this talk, we provide a proof of Solomon's Theorem as well as a counterexample for the modular case.